Python 3.12 floor: finish package metadata and compatibility cleanup #390

Reconciliation at main e222606

Partially complete; keep open for the breaking package-floor change. #397 (2944b1e) reduced the CI matrix to Python 3.12, and #399 (1dd222f) added xdist. The retired 3.10/3.11 required checks were removed with owner approval; a live protection read confirms only test (3.12) among the required Python contexts.

The package still declares requires-python = ">=3.10" and 3.10/3.11 classifiers. Compatibility branches/backports remain intentionally supported by the package metadata. The SDK security upgrade did not change that floor.

Remaining work: choose the minor/major release for the breaking >=3.12 floor, update package metadata/classifiers and compatibility code together, document the change, then validate the new minimum. Do not count the CI-only change as completion of this issue. Remaining stale CI wording is tracked separately in #398.

Original proposal and historical starting state

Current state:

• pyproject.toml:11 declares >=3.10, with classifiers 3.10/3.11/3.12.
• .github/workflows/test.yml:26 runs a three-version matrix, ["3.10", "3.11", "3.12"].

pmcp is published to PyPI. If it has outside users, a 3.12 floor is a breaking change for the next minor release (item 4). Even so, the CI matrix can drop to 3.12 now.

Why: these are internal repos, and right now we are effectively their only users. Testing and supporting Python versions older than 3.12 costs CI minutes and wall-clock time on every run, and it forces compatibility code (shims, version branches, backports) that nobody actually uses. Most of our CI already runs on 3.12, so 3.12 is a free floor. Consiliency/spec adopted it on 2026-10-12 (see the CHANGELOG "Unreleased" breaking note there).

Proposal: support Python 3.12+ only.

1. Set requires-python = ">=3.12" in every first-party pyproject.toml, and drop Programming Language :: Python :: 3.8–3.11 classifiers if present.
2. Collapse CI Python matrices to a single 3.12 entry, and move any single-version 3.10/3.11 jobs to 3.12.
3. Remove version shims and branches that only exist for <3.12 (sys.version_info < (3, 11) checks, tomli/typing_extensions backports, -P fallbacks).
4. If the package is published to PyPI, raising requires-python is a breaking change. Do it in the next minor or major release and note it in the CHANGELOG. pip already skips the new release on old interpreters, so existing pins keep working.

Acceptance: CI stays green on 3.12, fewer jobs run per push, and no first-party pyproject.toml declares a floor below 3.12.

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