test against all supported python versions via github actions

[venthur]

This PR adds github actions, to test on each pull and pull request against all supported python versions. For each version we run the respective tox -e XX and the flake8.

I took the liberty to remove all Python2.x versions, except the last one 2.7, and I removed all python3.x versions until 3.5 and added the most recent ones. So the currently supported versions are: 2.7, 3.5..3.9, pypy

I also removed the unneeded pypy-testenv in tox.ini which was identical with the default testenv for all other python versions.

I had to add a few flake8-exceptions due to the dual Python2/3 support. I suggest at some point to remove Python2 support completely.

All tests and flake8 runs currently pass on gitlab-ci with the supported python versions from above.