Announce support for Python 3.7

[florisla]

bump2version does not mention Python v3.7 support on PyPI, even though it works fine without any changes.

This commit adds the classifier Programming Language :: Python :: 3.7 and tells Travis/Tox/AppVeyor to test against 3.7 too.

[ekohl]

Why does this need sudo?

[florisla]

It's due to this Travis oddity: travis-ci/travis-ci#10110 .

[ekohl]

I don't mind moving the entire matrix to xenial. That should still support all the python versions.

[florisla]

Apparently Travis xenial does not support regular pypy / pypy3; I've kept those on trusty. Relevant issue is https://travis-ci.community/t/pypy-2-7-on-xenial/889 .

[c4urself]

In future, we should also be bumping the Dockerfile as part of this, (this was put in place so people can run the test suite on all versions locally via make test). I've already added the change for 3.7 to be installed here: 5f6d8f7 so the PRs are perfectly compatible.

[ekohl]

Thanks for the effort!

[florisla]

I've documented the pypy-on-trusty and sudo-on-xenial oddities in .travis.yml itself.

[c4urself]

I wonder if Travis let's you just run some Docker containers? If so, we may be able to bring make test into .travis.yml and make testing a bit more consistent?

[florisla]

Rebased on master.

[ekohl]

You can run docker containers on Travis, but I'd prefer to avoid that unless we have to.