Add Python 3.15 to the test workflow

[derek73]

Python 3.15.0 final is scheduled for 2026-10-01 (PEP 790; rc1 2026-08-04). actions/setup-python already carries pre-release builds (3.15.0-beta.3 as of July 2026), so the workflow can start testing it any time.

Two-step plan, whichever release picks this up:

1. Now-ish (any release): add a non-blocking pre-release job to python-package.yml so interpreter breakage surfaces early:

   - uses: actions/setup-python@v5
     with:
       python-version: "3.15"
       allow-prereleases: true

   with continue-on-error: true on the matrix entry so a beta-only failure warns without blocking merges.

2. After 3.15.0 final (October 2026): promote it to a regular blocking matrix entry, add the Programming Language :: Python :: 3.15 classifier, and drop continue-on-error.

Timing note: 3.15 final and Python 3.10 EOL land the same month, so if this coincides with 2.0, step 2 and the #257 floor bump (>=3.11) are one release-prep item — the supported window becomes exactly 3.11–3.15.

No milestone yet — undecided which release takes it.

[derek73]

Step 1 is running on the 2.0 branch (#288, commit a2d88ea): CI now includes a 3.15 pre-release job (allow-prereleases: true), non-blocking via continue-on-error so a beta regression can't turn the branch red while 2.0 is being built.

Current status: 3.15.0b3 is fully green — the complete suite, mypy, ruff, sdist build, twine check, and both Sphinx builds all pass, verified both locally and in CI.

Setting this up surfaced a pre-existing bug worth recording here: the matrix was decorative. uv sync honors the repo's .python-version file over the interpreter actions/setup-python provides, so every matrix job was silently testing the same Python (3.11 on the 2.0 branch — "build (3.14)" ran the suite on CPython 3.11.15, confirmed in the run logs). A job-level UV_PYTHON: ${{ matrix.python-version }} makes the matrix real; the same fix is needed on master, where all jobs currently test 3.10 (will land separately).

Remaining (step 2, when 3.15 goes final 2026-10-01): flip the job to blocking, add the Programming Language :: Python :: 3.15 classifier — pairing with #257's floor bump, which is already on the 2.0 branch.

[derek73]

Step 1 shipped in 2.0.0: 3.15 runs non-blocking with allow-prereleases (plus the UV_PYTHON fix so the matrix interpreter is actually used). Retargeting to v2.1 for step 2 — promote to blocking and add the 3.15 classifier when 3.15 goes final (October 2026).