Replace Travis CI with GitHub Actions

[hugovk]

Travis CI is no longer working, so I've replaced it with GitHub Actions.

Here's a sample build: https://github.com/hugovk/pytracery/actions/runs/12341947658/job/34441209716

I set it to only test the upstream supported versions, which are 3.9+, and also 3.14 alpha:

https://devguide.python.org/versions/

I think GitHub Actions has 3.8 available, but not 3.7 or lower, so we can add 3.8 if you like.

---

I tried adding Coveralls like this:

      - name: Upload coverage
        run: |
          uvx coveralls
        env:
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}"

But I got 500 errors on CI and also when trying to sign in, so I left it out.

I usually use Codecov as a coverage website, so we can add that if you like. In the meantime, coverage is reported in the CI terminal:

Name                      Stmts   Miss  Cover
---------------------------------------------
tests/test_modifiers.py     131      0   100%
tests/test_tracery.py       181      0   100%
tracery/__init__.py         283     15    95%
tracery/modifiers.py         38      0   100%
---------------------------------------------
TOTAL                       633     15    98%