Replace travis and tox with github actions

[Akuli]

I found tox quite confusing. It outputs a long wall of text when I run tox. I'd much rather have one local development environment, and have github actions try several different environments for me. If someone wants to keep tox, let me know and I'll try to preserve it.

[srittau]

I'm not using tox myself, so I'm +0 on removing it. And obviously +1 on dropping Travis CI (which doesn't work anymore) in favor of GitHub Actions. But currently this PR does a bit too much for me, since I'd like to keep a clean git history. Could we split at least the following?

• Fixing pyi.py and the tests, setup.py fixes
• Replacing Travis CI and tox with GitHub Actions

[Akuli]

Unrelated things should be gone now.

[JelleZijlstra]

Thank you!