#30214 (Parallelized Build / CI) caused a build failure for me
Currently I can't get pandas build.

I get the message when running ``python setup.py build_ext --inplace -j 4``:

```
Compiling pandas\_libs/parsers.pyx because it changed.
Compiling pandas\_libs/tslibs/timestamps.pyx because it changed.
Compiling pandas\_libs/window/aggregations.pyx because it changed.
Compiling pandas\io/msgpack/_packer.pyx because it changed.
Compiling pandas\io/msgpack/_unpacker.pyx because it changed.
Compiling pandas\_libs/groupby.pyx because it changed.
Compiling pandas\_libs/index.pyx because it changed.
Compiling pandas\_libs/internals.pyx because it changed.
Compiling pandas\_libs/lib.pyx because it changed.
Compiling pandas\_libs/parsers.pyx because it changed.
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 105, in spawn_main
Compiling pandas\_libs/tslibs/timestamps.pyx because it changed.
Compiling pandas\_libs/window/aggregations.pyx because it changed.
Compiling pandas\io/msgpack/_packer.pyx because it changed.
Compiling pandas\io/msgpack/_unpacker.pyx because it changed.
Compiling pandas\_libs/groupby.pyx because it changed.
Compiling pandas\_libs/index.pyx because it changed.
Compiling pandas\_libs/internals.pyx because it changed.
Compiling pandas\_libs/lib.pyx because it changed.
Compiling pandas\_libs/parsers.pyx because it changed.
    exitcode = _main(fd)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 114, in _main
    prepare(preparation_data)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 225, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 277, in _fixup_main_from_path
    run_name="__mp_main__")
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\runpy.py", line 263, in run_path
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 105, in spawn_main
    exitcode = _main(fd)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 114, in _main
    prepare(preparation_data)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 225, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 277, in _fixup_main_from_path
    run_name="__mp_main__")
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\runpy.py", line 263, in run_path
Compiling pandas\_libs/tslibs/timestamps.pyx because it changed.
Compiling pandas\_libs/window/aggregations.pyx because it changed.
Compiling pandas\io/msgpack/_packer.pyx because it changed.
Compiling pandas\io/msgpack/_unpacker.pyx because it changed.
    pkg_name=pkg_name, script_name=fname)
    pkg_name=pkg_name, script_name=fname)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\runpy.py", line 96, in _run_module_code
    mod_name, mod_spec, pkg_name, script_name)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\runpy.py", line 85, in _run_code
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\runpy.py", line 96, in _run_module_code
    exec(code, run_globals)
  File "C:\Users\TP\Documents\Python\pandasdev\pandasdev\setup.py", line 815, in <module>
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 105, in spawn_main
    mod_name, mod_spec, pkg_name, script_name)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Users\TP\Documents\Python\pandasdev\pandasdev\setup.py", line 815, in <module>
    exitcode = _main(fd)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 114, in _main
    ext_modules=maybe_cythonize(extensions, compiler_directives=directives),
  File "C:\Users\TP\Documents\Python\pandasdev\pandasdev\setup.py", line 543, in maybe_cythonize
    prepare(preparation_data)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 225, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 277, in _fixup_main_from_path
    run_name="__mp_main__")
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\runpy.py", line 263, in run_path
    return cythonize(extensions, *args, **kwargs)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\site-packages\Cython\Build\Dependencies.py", line 1073, in cythonize
    pkg_name=pkg_name, script_name=fname)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\runpy.py", line 96, in _run_module_code
    ext_modules=maybe_cythonize(extensions, compiler_directives=directives),
  File "C:\Users\TP\Documents\Python\pandasdev\pandasdev\setup.py", line 543, in maybe_cythonize
    mod_name, mod_spec, pkg_name, script_name)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\runpy.py", line 85, in _run_code
    return cythonize(extensions, *args, **kwargs)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\site-packages\Cython\Build\Dependencies.py", line 1073, in cythonize
    exec(code, run_globals)
  File "C:\Users\TP\Documents\Python\pandasdev\pandasdev\setup.py", line 815, in <module>
    ext_modules=maybe_cythonize(extensions, compiler_directives=directives),
    nthreads, initializer=_init_multiprocessing_helper)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\context.py", line 119, in Pool
  File "C:\Users\TP\Documents\Python\pandasdev\pandasdev\setup.py", line 543, in maybe_cythonize
    nthreads, initializer=_init_multiprocessing_helper)
    return cythonize(extensions, *args, **kwargs)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\site-packages\Cython\Build\Dependencies.py", line 1073, in cythonize
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\context.py", line 119, in Pool
    nthreads, initializer=_init_multiprocessing_helper)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\context.py", line 119, in Pool
Compiling pandas\_libs/groupby.pyx because it changed.
Compiling pandas\_libs/index.pyx because it changed.
    context=self.get_context())
    context=self.get_context())
    context=self.get_context())
Compiling pandas\_libs/internals.pyx because it changed.
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\pool.py", line 176, in __init__
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\pool.py", line 176, in __init__
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\pool.py", line 176, in __init__
Compiling pandas\_libs/lib.pyx because it changed.
Compiling pandas\_libs/parsers.pyx because it changed.
Compiling pandas\_libs/tslibs/timestamps.pyx because it changed.
Compiling pandas\_libs/window/aggregations.pyx because it changed.
Compiling pandas\io/msgpack/_packer.pyx because it changed.
Compiling pandas\io/msgpack/_unpacker.pyx because it changed.
    self._repopulate_pool()
    self._repopulate_pool()
    self._repopulate_pool()
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\pool.py", line 241, in _repopulate_pool
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\pool.py", line 241, in _repopulate_pool
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\pool.py", line 241, in _repopulate_pool
Traceback (most recent call last):
    w.start()
    w.start()
    w.start()
  File "<string>", line 1, in <module>
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\process.py", line 112, in start
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\process.py", line 112, in start
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\process.py", line 112, in start
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 105, in spawn_main
    exitcode = _main(fd)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 114, in _main
    prepare(preparation_data)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 225, in prepare
    _fixup_main_from_path(data['init_main_from_path'])
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 277, in _fixup_main_from_path
    run_name="__mp_main__")
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\runpy.py", line 263, in run_path
    pkg_name=pkg_name, script_name=fname)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\runpy.py", line 96, in _run_module_code
    mod_name, mod_spec, pkg_name, script_name)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "C:\Users\TP\Documents\Python\pandasdev\pandasdev\setup.py", line 815, in <module>
    ext_modules=maybe_cythonize(extensions, compiler_directives=directives),
  File "C:\Users\TP\Documents\Python\pandasdev\pandasdev\setup.py", line 543, in maybe_cythonize
    return cythonize(extensions, *args, **kwargs)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\site-packages\Cython\Build\Dependencies.py", line 1073, in cythonize
    nthreads, initializer=_init_multiprocessing_helper)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\context.py", line 119, in Pool
    context=self.get_context())
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\pool.py", line 176, in __init__
    self._repopulate_pool()
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\pool.py", line 241, in _repopulate_pool
    w.start()
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\process.py", line 112, in start
    self._popen = self._Popen(self)
    self._popen = self._Popen(self)
    self._popen = self._Popen(self)
    self._popen = self._Popen(self)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\context.py", line 322, in _Popen
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\context.py", line 322, in _Popen
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\context.py", line 322, in _Popen
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\context.py", line 322, in _Popen
    return Popen(process_obj)
    return Popen(process_obj)
    return Popen(process_obj)
    return Popen(process_obj)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\popen_spawn_win32.py", line 46, in __init__
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\popen_spawn_win32.py", line 46, in __init__
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\popen_spawn_win32.py", line 46, in __init__
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\popen_spawn_win32.py", line 46, in __init__
    prep_data = spawn.get_preparation_data(process_obj._name)
    prep_data = spawn.get_preparation_data(process_obj._name)
    prep_data = spawn.get_preparation_data(process_obj._name)
    prep_data = spawn.get_preparation_data(process_obj._name)
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 143, in get_preparation_data
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 143, in get_preparation_data
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 143, in get_preparation_data
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 143, in get_preparation_data
    _check_not_importing_main()
    _check_not_importing_main()
    _check_not_importing_main()
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 136, in _check_not_importing_main
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 136, in _check_not_importing_main
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 136, in _check_not_importing_main
    _check_not_importing_main()
    is not going to be frozen to produce an executable.''')
RuntimeError:
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.    is not going to be frozen to produce an executable.''')
  File "C:\Users\TP\Miniconda3\envs\pandas-dev\lib\multiprocessing\spawn.py", line 136, in _check_not_importing_main
    is not going to be frozen to produce an executable.''')

RuntimeError:
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...

        The "freeze_support()" line can be omitted if the program
        is not going to be frozen to produce an executable.    is not going to be frozen to produce an executable.''')
RuntimeError:
        An attempt has been made to start a new process before the
        current process has finished its bootstrapping phase.

        This probably means that you are not using fork to start your
        child processes and you have forgotten to use the proper idiom
        in the main module:

            if __name__ == '__main__':
                freeze_support()
                ...
```

I can't really parse what's going on, but the build seems to go back to the same RuntimeError repeatedly.

I've triaged the issue to stem from #30214. Any idea what's happening, @WillAyd ?

## Workaround

I can work around the issue by building with parallelization, i.e. run ``python setup.py build_ext --inplace -j 0`` instead of ``python setup.py build_ext --inplace -j 4``.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : 95e1a63dd3382db6663bc8a2b334b422b93dd7fe
python           : 3.7.5.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
machine          : AMD64
processor        : Intel64 Family 6 Model 78 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 0.26.0.dev0+1358.g95e1a63dd
numpy            : 1.17.4
pytz             : 2019.1
dateutil         : 2.8.0
pip              : 19.3.1
setuptools       : 42.0.2.post20191203
Cython           : 0.29.13
pytest           : 5.2.2
hypothesis       : 4.28.2
sphinx           : 2.2.0
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.6.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : None
matplotlib       : 3.1.1
numexpr          : 2.6.9
odfpy            : None
openpyxl         : 3.0.0
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : 5.2.2
s3fs             : None
scipy            : 1.3.1
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
</details>
