GeezerVison

Fixes #16499.

Description

In #16499, wrapping a callable in partialmethod resulted in partialmethod[Any] in mypy (e.g. partialmethod(pprint, options={}) where pprint -> str).

This was caused by the secondary func: _Descriptor overload where _Descriptor: TypeAlias = Any. Because _Descriptor had no _T type parameter, mypy unified every callable argument against both overloads, joining _T with Any and losing return type information.

Align partialmethod with partial (which does not have an unconstrained descriptor overload):

• Remove the unconstrained func: _Descriptor overload on __init__ / __new__
• Fix the first parameter name of __new__ from self to cls
• Add regression test in stdlib/@tests/test_cases/check_functools.py asserting partialmethod and instance calls preserve _T.