Migrate to GitHub Actions

[rhysd]

Fix #386

Since travis-ci.org no longer works, this PR migrates CI to GitHub Actions.

• I dropped Python 3.7 or earlier because they are no longer supported officially
• I added Python 3.10
• I skipped test_exec_vint_with_color_flag test. It does not pass probably because GitHub Actions runs commands in non-interactive shell. In the case vint doesn't seem to enable color even if --color is set
• I dropped coveralls and used only codecov for simplify CI steps

I confirmed the workflow passed on my fork repository: https://github.com/rhysd/vint/actions/runs/3290301039

[rhysd]

@blueyed I think a maintainer needs to do the following actions as well as merging this PR.

• Could you visit https://codecov.io/gh/Vimjas/vint after login to codecov? Then the repository will be activated and coverage measurements can be monitored there
• Please stop Travis CI workflow not to put a status check in PR page

[blueyed]

• Could you visit https://codecov.io/gh/Vimjas/vint after login to codecov? Then the repository will be activated and coverage measurements can be monitored there

Activated.

• Please stop Travis CI workflow not to put a status check in PR page

Edited the branch protection rules.

[blueyed]

• I skipped test_exec_vint_with_color_flag test. It does not pass probably because GitHub Actions runs commands in non-interactive shell. In the case vint doesn't seem to enable color even if --color is set

I have not looked at the code, but a missing TERM appears to trigger it already, see env -u TERM vint --color ./test/fixture/cli/invalid1.vim.

This likely happens throuth ansicolor: https://github.com/numerodix/ansicolor/blob/fbbeaee3a446f4520b94d12f007f98f94ac319ec/ansicolor/ansicolor.py#L37

[rhysd]

Thank you for the point. I passed TERM environment variable at 2ed2b35 and confirmed the workflow passed.

https://github.com/rhysd/vint/actions/runs/3296982905

[blueyed]

Thank you!