👷 ci: test against Python 3.15

[gaborbernat]

Python 3.15 is at 3.15.0b3. This extension reproduces argparse output in the rendered docs, so a change to the upstream formatter turns into a diff in every project that uses it. 🐍 Running the suite against the beta gives us a few months to react before 3.15 ships.

I added 3.15 and 3.15t to the env matrix in check.yaml, plus the matching env_list entries in tox.toml so the environments exist locally too. Both follow how 11cf41f declared 3.14. Neither needs a prerelease flag, since uv python install and the tox interpreter discovery pick up the newest available build.

pyproject.toml gains the Programming Language :: Python :: 3.15 classifier, and [tool.pyproject-fmt] max_supported_python moves to 3.15. Without that second entry the hook strips the classifier as newer than the Python it knows, so the two go together. The entry comes back out once pyproject-fmt's own maximum catches up, the way the 3.13 one did. requires-python is unchanged and no version is dropped.