Initial GitHub actions support

[penguinolog]

Checklist

• I've ensured that similar functionality has not already been implemented
• I've ensured that similar functionality has not earlier been proposed and declined
• I've branched off the master or python-dual-support branch
• I've merged fresh upstream into my branch recently

[penguinolog]

This PR expect, that #517 and #522 already merged

[penguinolog]

Example run of exactly this PR:
https://github.com/penguinolog/urwid/actions/runs/4545199541

[penguinolog]

https://github.com/penguinolog/urwid/actions/runs/4545811232 - run after test fix merged.
@wardi , can I merge it?

[penguinolog]

Validated:
travis-ci.org: error 503
travis-ci.com: project not migrated
CI for pull requests is not available without this PR.
Test CI run for PR with last changes: https://github.com/penguinolog/urwid/actions/runs/4547105922
Doctest fail was not visible on local prior to merge and run on github