Test on Python 3.15, and fix the typing._eval_type call it breaks

[wbarnha]

Description

Adds Python 3.15 to the CI matrix, and fixes the one thing that stopped the suite passing on it.

CI

Two new rows — 3.15 and 3.15t (default and free-threaded builds), matching the coverage the 3.14 rows get.

allow-prereleases: true on setup-python resolves 3.15 to whatever pre-release the runner image publishes — 3.15.0rc1 today — and will pick up 3.15.0 final in October with no further change here. Setting it does not disturb the stable rows: it widens 3.X to ~3.X.0-0, and a pre-release only wins when no stable release satisfies the spec, so 3.10–3.14 still resolve to their newest stable patch. I verified this against the real actions/python-versions manifest — 3.10 → 3.10.20, 3.12 → 3.12.13, 3.14 → 3.14.7, identical with and without the flag; and without it 3.15 resolves to nothing at all, which is the "Version 3.15 was not found in the local cache" failure.

The new rows are advisory (experimental: true → continue-on-error) while 3.15 is a pre-release, so a CPython-side regression in rc2 or final can't block merges on a version nothing runs in production. The comment in the workflow records how to promote them to required once 3.15.0 ships.

The fix

The suite did not pass as-is. typing._eval_type gained a type_params argument in 3.13, where omitting it only emitted a DeprecationWarning and defaulted to (). In 3.15 it is a required positional, so every call raised:

TypeError: _eval_type() missing 1 required positional argument: 'type_params'

That took out all five annotation tests here. It also stopped faust importing at all on 3.15 — every faust Record model resolves its annotations through mode.utils.objects.annotations, so collection died before the first test ran. See faust-streaming/faust#770, which is blocked on this landing and being released.

Nothing resolved through eval_type carries PEP 695 type parameters of its own, so () is both what 3.13/3.14 already substituted when the argument was omitted and what 3.15 requires — passing it explicitly keeps behaviour identical across versions and silences the 3.13/3.14 deprecation warning on the way.

The signature is inspected once at import rather than caught per call on purpose: _eval_type also raises TypeError for annotations typing._type_check rejects, and eval_type depends on being able to tell those two apart (that's the ClassVar branch right below it).

Verification

Ran against the actual 3.15.0rc1 build the runners use, not a proxy:

	result
3.15.0rc1	866 passed, 1 skipped
3.15.0rc1 free-threaded (3.15t)	866 passed — GIL confirmed disabled
3.11 (regression check)	866 passed, unchanged
ruff check + ruff format --check	clean
mypy typecheck test	clean

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