fix: handle parameterized generics in field(type=...) to avoid infinite recursion

[gaoflow]

Summary

Fixes #291.

Passing a parameterized generic like tuple[int, int] (a types.GenericAlias) or
typing.List[int] (a typing._GenericAlias) as the type= argument to field()
caused maybe_parse_user_type to recurse forever and raise a RecursionError.

Root cause: Both alias kinds satisfy isinstance(t, Iterable), so the code
entered the iterable branch and iterated the alias. Iterating a
types.GenericAlias yields a starred version of itself (e.g. *tuple[int, int]),
which is itself a GenericAlias and also Iterable — the recursion never bottoms out.

Fix: Before falling through to the generic-iterable branch, detect objects that
carry an __origin__ attribute but are not plain type instances (i.e. all
GenericAlias forms). Call typing.get_origin() to extract the bare origin class
and return that. This makes field(type=tuple[int, int]) behave like
field(type=tuple) for runtime isinstance checks while correctly rejecting
non-tuple values.

Minimal reproducer (before fix → RecursionError; after fix → works)

from pyrsistent import PClass, field

class Coords(PClass):
    position = field(type=tuple[int, int])  # RecursionError before this fix

c = Coords(position=(1, 2))  # OK

Test

Added test_field_type_generic_alias in tests/class_test.py covering
tuple[int, int], list[str], and dict[str, int] aliases. Full test suite
(635 tests) passes.

This pull request was prepared with the assistance of AI, under my direction and review.