move off Travis.org to Github Actions CI

[anthrotype]

For example, here's how I ported another multibuild-based project to use Github Actions => https://github.com/fonttools/skia-pathops/blob/master/.github/workflows/ci.yml

[McSinyx]

Not entirely related, but CPython 3.9 was released and soon will 3.10.

[anthrotype]

a couple of PRs are currently stuck because of repo settings enforce all status checks to pass before allowing the merge, and the Travis CI check is stale and will never even start.
@mikekap As I was saying in #43, I and @madig can help transitioning the CI setup to Github Actions but it'd help if we gain admin access to the repository (e.g. by moving the repo under the fonttools organization, or by adding us as co-administrators to the personal repo).

[McSinyx]

Just in case you didn't notice that this repo has been moved under fonttools. I don't mean to push you or anything though.