DEPS/BLD: 3.9 builds breaking with missing Cython
xref https://github.com/pandas-dev/pandas/pull/43952/checks?check_run_id=3849531985 as an exxample

maybe something got released / renamed? looks like an issue with coverage

```
File "/usr/share/miniconda/envs/pandas-dev/lib/python3.9/site-packages/pytest_cov/plugin.py", line 126, in pytest_load_initial_conftests
    plugin = CovPlugin(options, early_config.pluginmanager)
  File "/usr/share/miniconda/envs/pandas-dev/lib/python3.9/site-packages/pytest_cov/plugin.py", line 175, in __init__
    self.start(engine.DistMaster)
  File "/usr/share/miniconda/envs/pandas-dev/lib/python3.9/site-packages/pytest_cov/plugin.py", line 199, in start
    self.cov_controller.start()
  File "/usr/share/miniconda/envs/pandas-dev/lib/python3.9/site-packages/pytest_cov/engine.py", line 44, in ensure_topdir_wrapper
    return meth(self, *args, **kwargs)
  File "/usr/share/miniconda/envs/pandas-dev/lib/python3.9/site-packages/pytest_cov/engine.py", line 267, in start
    self.cov.start()
  File "/usr/share/miniconda/envs/pandas-dev/lib/python3.9/site-packages/coverage/control.py", line 532, in start
    self._init()
  File "/usr/share/miniconda/envs/pandas-dev/lib/python3.9/site-packages/coverage/control.py", line 275, in _init
    self._plugins = Plugins.load_plugins(self.config.plugins, self.config, self._debug)
  File "/usr/share/miniconda/envs/pandas-dev/lib/python3.9/site-packages/coverage/plugin_support.py", line 41, in load_plugins
    __import__(module)
ModuleNotFoundError: No module named 'Cython'
```

cc @pandas-dev/pandas-core 