Reject non-set objects for concrete set annotations

[jakezwang]

Changes

check_type(frozenset({1}), set[int]) currently passes because concrete sets use the same instance check as AbstractSet. Dictionary key views are also accepted, although neither provides the mutable set API.

Require a set instance for concrete set annotations. Keep accepting set subclasses, and preserve the broader behavior of AbstractSet. Tests cover built-in and typing annotations, empty/nonempty frozen sets, key views, and subclass controls.

Validation: full tox suite on Python 3.10–3.15, pre-commit and package checks passed. The strict docs build has the same two typing.Union cross-reference warnings as unmodified master.

Checklist

• Added tests that fail without the patch.
• Updated the supported-type documentation.
• Added a changelog entry.

[coveralls]

coverage: 94.862% (+0.009%) from 94.853% — jakezwang:fix/concrete-set-type-check into agronholm:master