Consider switching CI to Github Workflows, include Windows

[roryyorke]

Nothing for this release, but would it be worth to switch from Travis to Github Workflows and include Windows builds there? Possibly based on #80, although to me that seems a bit outdated by now.

Originally posted by @bnavigator in #101

[bnavigator]

This will also be a good opportunity to adopt NEP 29, which effectively means to drop the support for Python 2 and 3.5 now and 3.6 very soon.

[bnavigator]

I have been trying to prepare something in #140, but it is not finished yet.

Dropping Python 2 will also help to avoid workarounds like in #139.