Replace typing.FrozenSet with frozenset

PEP 585 deprecated typing.FrozenSet in Python 3.9. Replace it with the built-in frozenset wherever a type is expected, including class base lists, where either spelling subclasses frozenset. Applies inside if TYPE_CHECKING: blocks as well; the built-in generic is valid at runtime from Python 3.9 on, so no from __future__ import annotations is needed. References in a value position are left alone: typing.FrozenSet and frozenset are distinct objects, so substituting one for the other where the name is used as a value would change behaviour.