ci: run the test suite on every supported Python version

[karpovantonme]

Describe your changes

pyproject.toml promises five Python versions, in the classifiers and in requires-python = ">=3.10,<4":

Programming Language :: Python :: 3.10
Programming Language :: Python :: 3.11
Programming Language :: Python :: 3.12
Programming Language :: Python :: 3.13
Programming Language :: Python :: 3.14

pytest.yaml pins python-version: '3.13', and there is no tox or nox picking up the rest. So four of the five promised versions are never exercised, and a 3.10-only break would ship unnoticed.

This adds a matrix over exactly the versions the classifiers list, with fail-fast: false so one red version does not hide the state of the others.

I ran the suite on all five before proposing this, so this is not a guess about whether they still work:

Python	Result
3.10	188 passed, 5 skipped
3.11	188 passed, 5 skipped
3.12	188 passed, 5 skipped
3.13	188 passed, 5 skipped
3.14	188 passed, 5 skipped

The support is real. It just was not guarded, which is the part this fixes.

On CI minutes: this turns one job into five, and the suite takes well under a minute, so it should be cheap. If that is still too much on every PR, the other shape is a full matrix on push to main and a single version on pull requests. Either works for me, your call.

No issue opened for this, since it is a one file change. Can open one if you track things that way.

Checklist:

• I have read and followed the contribution guide.
• I have performed a self-review of my code.
• I have added tests that prove my fix is effective or that my feature works. — this change is test configuration, there is nothing to add a test for
• I have successfully run all tests with make test. — run on all five versions, table above
• I have successfully run pre-commit with make pre-commit. — not run, this touches only a workflow file
• I have written documentation for the changes. — no user facing change
• I have rebased my branch onto the target branch (usually main).