BUG: tox_matrix.py shouldn't run with latest python #436

I previously introduced an upper bound in #410
It was removed in #428, addressing #427

however, I want to point out two things

• I don't understand what problem specifying python-version: 3 in setup-uv uses the runner's system Python instead of latest Python as expected #427 is describing: the host python running tox does not limit what versions can be used in individual tox envs, which is what I expect users to care about
• because neither uv itself nor python is pinned, users may get a version of Python they are not actually ready for, and which clashes with their configuration. I believe this is the root cause for what we are seeing in TST: CI failing because uv run tox_matrix.py cannot find pyyaml==6.0.2 wheels astropy/astropy#20715

Pinning uv itself would avoid the default python version changing silently, however it could also delay security updates. I suggest pinning the host python instead to 3.14, which is what has been used so far. Upgrades to this pin should be mentionned on the changelog in the future, and I don't recommend pushing for 3.15 before the ecosystem is largely supporting it.