convert travis to a github action

[StalkR]

Travis-CI is no more, replaced by GitHub actions which are free for OSS projects like pyformat.

I propose replacing the old .travis.yml with an equivalent github action workflow. Small differences:

• tested versions were 2.7, 3.4-6, now 3.8-14
  • this is because 2.7 is no longer supported, entirely dropped by actions/setup-python
  • and version below 3.8 are not available for the runs-on: ubuntu-latest which is ubuntu24.04, cf. python versions-manifest.json
  • if desired, supporting older releases is possible, it would require specifying older os releases (e.g. ubuntu22.04 for 3.7, all the way to ubuntu18.04 for 3.4), but probably not worth the headache
• using the recommended pip install -e . instead of the deprecated and broken python setup.py install
• posting coverage to coveralls as before, but using their coverallsapp/github-action

The github action is configured to be run:

• automatically on pull requests: this allows author and maintainers to be sure a change doesn't break the build and tests
• automatically on pushes: in case of changes pushed outside of a pull request, this ensures every push also runs it, to avoid mistakenly breaking the build and tests
• manually: on demand via the github actions page by clicking 'run workflow'

It only runs on linux. It could be configured to also run on windows/macos but I don't think it matters for this project.

Example run: https://github.com/StalkR/pyformat/actions/runs/19685648150

All builds and tests pass, except 3.14 which is currently broken, and for which I've sent a separate PR to fix: #14 (once merged, the tests are expected to pass).

I also updated the README.rst to update the build badge (from Travis-CI to GitHub Actions) and add the coveralls badge.

Future work could be a GitHub Action to automate publishing the package to PyPI.

Thank you for maintaining this project and I hope this helps!