Support Python 3.7 and 3.8, drop 3.4

[ekohl]

I didn't drop 3.4 from setup.py for now since there was no actual code change, but I don't know about the actual policy for this repo.

[msiemens]

Thanks for your PR! For setup.py I usually just list the versions that are used for CI tests. Using PyGitUp with older versions may work but there are no guarantees. If you remove the setup.py line, I'll be happy to merge this 🙂

[ekohl]

That makes sense and I'd probably do the same. I've updated the commits to also update README.rst.

It makes me wonder what you think about dropping Python 2.7.

[msiemens]

Thanks @ekohl, this is now merged :)