[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/openapi3/server/petstore/python-flask-python2/test-requirements.txt

@@ -1,6 +1,4 @@
-flask_testing==0.6.1
-coverage>=4.0.3
-nose>=1.3.7
-pluggy>=0.3.1
-py>=1.4.31
-randomize>=0.13
+pytest~=4.6.7 # needed for python 2.7+3.4
+pytest-cov>=2.8.1
+pytest-randomly==1.2.3 # needed for python 2.7+3.4
+flask_testing==0.6.1