Investigate dropping runtime PDM dependency

[ncoghlan]

uv 0.3 release announcement: https://astral.sh/blog/uv-unified-python-packaging

uv python install may be able to replace the use of pdm and pbs-installer for Python runtime installation.

However, we'll need something equivalent to the dev version pinning on pbs-installer to keep the runtime selection in CI stable.

[ncoghlan]

Checking https://docs.astral.sh/uv/concepts/python-versions/#python-implementation-support I get the impression that a version pin on uv itself will serve the same purpose as the pbs-installer pin that's currently implemented.

uv is already explicitly pinned in CI to avoid unexpected changes to the format of exported lockfiles, so that shouldn't pose any problems.

[ncoghlan]

venvstacks has wrapped the pbs-installer download step to apply the tar extraction filter on CPython 3.12+.

If uv already implements similar tarball filtering, that would be an additional reason to switch (since we'd also get the filtering on 3.11), while if it doesn't, it would be a reason to delay switching (since we'd lose the filtering on 3.12+)