Replace Travis CI and Coveralls with GitHub Actions and CodeCov

[hugovk]

Travis CI have changed their pricing model making it harder to use with open source projects, and also builds stopped 9 months ago, and many projects have switched to GitHub Actions.

A big advantage of GHA is, in addition to Ubuntu, we can test on macOS and Windows. And instead of 5 parallel jobs, we get 20.

This does that, and also switches to CodeCov from Coveralls, as it's easier to integrate.

Sample build

https://github.com/hugovk/gpxpy/actions/runs/1631499293