Add Python 3.15 test infrastructure

[JelleZijlstra]

• add Python 3.15 to the stdlib mypy, regression-test, and stdlib stubtest matrices
• add Python 3.15 stdlib stubtest allowlists, including platform-specific allowlists for CI
• move allowlist entries that are no longer needed on 3.15 out of the shared allowlist and into 3.10-3.14 allowlists
• skip protobuf-sync and baseline-generation helper dependencies on Python 3.15 while their dependency stacks are not ready
• in the Python 3.15 mypy and regression-test jobs, skip only the named third-party stubs whose runtime dependency stacks do not install on 3.15 yet: numpy-dependent packages, jsonschema -> referencing -> rpds-py/PyO3, and seaborn -> matplotlib -> contourpy

Not yet enabled: the pyright 3.15 CI leg, because its bulk third-party dependency installation currently pulls dependency stacks that are not installable on Python 3.15 yet, including rpds-py/PyO3.

[JelleZijlstra]

Codex is still chugging along but I think this is close enough that it can be reviewed (current CI has one failure but it's a flake).

The main thing I'm uncertain about is the testing for third-party packages. I had it add a denylist of packages that have deps that don't support 3.15 yet and exclude them from mypy; @srittau do you think that's the right approach? We could also skip 3.15 CI completely for now for third-party packages, as it feels less valuable than for the stdlib.

I think the current iteration also skips pyright 3.15 CI completely, I'll make that work similarly to the mypy CI, whatever we decide on.

[srittau]

I think in the past we skipped third-party tests only for those packages that didn't work yet, although I don't recall how exactly we did that. I'm fine for skipping all third-party tests for now.

[JelleZijlstra]

For pyright we now skip everything (because it's harder to exclude just some packages), for mypy only those exact packages that are broken. That seems fine for now.

[JelleZijlstra]

Apparently Ubuntu 3.15 on CI doesn't have the C version of decimal which seems a bit concerning.

[srittau]

The Python versions on GitHub are sometimes a bit concerning ...

[srittau]

Looks good, just two comment-related remarks.

[srittau]

Could we add a TODO comment to all new allowlists?

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

pandas (https://github.com/pandas-dev/pandas)
+ pandas/core/computation/ops.py:328: error: Need type annotation for "_binary_ops_dict" (hint: "_binary_ops_dict: dict[<type>, <type>] = ...")  [var-annotated]