PYTHON-694 Test mod_wsgi sub interpreters

[ShaneHarvey]

https://jira.mongodb.org/browse/PYTHON-694

Tasks:

• Tests mod_wsgi sub interpreters.
• Tests mod_wsgi embedded mode.
• Test decoding all BSON types.
• Test encoding all BSON types.
• Test that decoded types created from C are valid in Python, ie assert isinstance(doc["regex"], Regex).
• Update Readme to document why multiple URLs are used.
• Fix mypy error:

      test/mod_wsgi_test/mod_wsgi_test.py:71: error: Cannot find implementation or
library stub for module named "mod_wsgi"  [import]
        from mod_wsgi import version as mod_wsgi_version

References:

• https://emptysqua.re/blog/python-c-extensions-and-mod-wsgi/
• https://modwsgi.readthedocs.io/en/develop/user-guides/checking-your-installation.html?#sub-interpreter-being-used

[ShaneHarvey]

Somewhat confusingly, these tests pass even with DatetimeMS which should be expected to fail due to the bug described in https://jira.mongodb.org/browse/PYTHON-3847. I'm a bit perplexed here but we can merge this work regardless. Perhaps Python or mod_wsgi was changed to make Python classes work between sub interpreters?

[blink1073]

Nice work!