[Python] Updates python test framework to pytest (#4765)

* Switches python generators to use pytest, useNose CLI option added if to allow nose to be used instead

* Adds ensure-up-to-date changes

* Adds setup.cfg to python clients so we can configure nose when useNose=true

* Adds fix for python-aiohttp testing, adds files missing from ensure-up-to-date

samples/server/petstore/python-flask/tox.ini

@@ -6,5 +6,4 @@ deps=-r{toxinidir}/requirements.txt
      -r{toxinidir}/test-requirements.txt
 
 commands=
-   nosetests \
-      []
+   pytest --cov=openapi_server