NameError: name 'tempita' is not defined when building extensions with old cython
#### Code Sample, a copy-pastable example if possible

```python
pietro@debiousci:~/nobackup/repo/pandas_temp$ python3 setup.py build_ext --inplace 
Traceback (most recent call last):
  File "setup.py", line 813, in <module>
    ext_modules=maybe_cythonize(extensions, compiler_directives=directives),
  File "setup.py", line 541, in maybe_cythonize
    build_ext.render_templates(_pxifiles)
  File "setup.py", line 127, in render_templates
    pyxcontent = tempita.sub(tmpl)
NameError: name 'tempita' is not defined
```

#### Problem description

If cython is too old/missing, the nice error reporting of the setup.py do not work.

Until few minutes ago I had never heard about ``tempita``, but my understanding is that it is required (used in ``build_ext.render_templates``) while ``cython`` is not:
https://github.com/pandas-dev/pandas/blob/bee17d5f4c99e6d1e451cf9a7b6a1780aa25988d/setup.py#L74

Hence, whether the check for ``tempita`` is made should not depend on whether ``cython`` is found: instead, this is what currently happens, given that ``tempita`` is checked in the ``else`` clause of the ``try...except`` check of ``cython``:
https://github.com/pandas-dev/pandas/blob/bee17d5f4c99e6d1e451cf9a7b6a1780aa25988d/setup.py#L76

Notice that this means the above error pops out despite ``tempita`` being installed in my system.

#### Expected Output

None

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

<details>

Commit: bee17d5f4c99e6d1e451cf9a7b6a1780aa25988d

</details>
