Support Python 3.8-3.10, drop EOL 2.7 and 3.5, replace Travis CI with GitHub Actions

[hugovk]

Travis CI is being turned off for the pytest-dev org (internal discussion), due to the recent security handling (1, 2).

This PR replaces Travis CI with GitHub Actions and replicates the old Travis workflow.

In addition to testing on Linux, it also tests on macOS and Windows.

Note: for some reason Python 3.7-3.10 are failing on Windows so they've been excluded from the CI for now.

This PR also adds support for Python 3.8 to 3.10 and drops support for EOL 2.7 and 3.5.

[hugovk]

Sample build:

• https://github.com/hugovk/unittest2pytest/runs/3676695636?check_suite_focus=true