Test against the new type of interpreter in Python 3.14

[namurphy]

The Python v3.14.0a7 release notes indicate that there will be a new type of interpreter that improves performance by a geometric mean of ∼3–5% with the current implementation. This interpreter is opt-in for now, and "doesn't change the visible behavior of Python programs at all."

After Python 3.14 is released in October 2025, it would be worthwhile to run tests using this new type of interpreter. It would be sensible to run it in CI as the Python 3.14 test because it will make sure that PlasmaPy works with the new interpreter (and potentially future versions of Python) and will let us get feedback from CI a little bit quicker.

I suspect that we will be able to make this change quickly with uv if it provides the new interpreter as one of the versions of Python that can be installed.