Simplify noxfile setup. [(#2806)](#2806)

* chore(deps): update dependency requests to v2.23.0

* Simplify noxfile and add version control.

* Configure appengine/standard to only test Python 2.7.

* Update Kokokro configs to match noxfile.

* Add requirements-test to each folder.

* Remove Py2 versions from everything execept appengine/standard.

* Remove conftest.py.

* Remove appengine/standard/conftest.py

* Remove 'no-sucess-flaky-report' from pytest.ini.

* Add GAE SDK back to appengine/standard tests.

* Fix typo.

* Roll pytest to python 2 version.

* Add a bunch of testing requirements.

* Remove typo.

* Add appengine lib directory back in.

* Add some additional requirements.

* Fix issue with flake8 args.

* Even more requirements.

* Readd appengine conftest.py.

* Add a few more requirements.

* Even more Appengine requirements.

* Add webtest for appengine/standard/mailgun.

* Add some additional requirements.

* Add workaround for issue with mailjet-rest.

* Add responses for appengine/standard/mailjet.

Co-authored-by: Renovate Bot <[email protected]>

translation/samples/snippets/requirements-test.txt

@@ -1,3 +1,3 @@
 backoff==1.10.0
 flaky==3.7.0
-pytest==5.4.3
+pytest==5.4.3