[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/client/petstore/python-experimental/test_python2.sh

@@ -19,11 +19,9 @@ fi
 
 ### install dependencies
 pip install -r $REQUIREMENTS_FILE | tee -a $REQUIREMENTS_OUT
-pip install -r test-requirements.txt
-python setup.py develop
 
 ### run tests
-nosetests || exit 1
+tox -e py27 || exit 1
 
 ### static analysis of code
 flake8 --show-source petstore_api/