gh-156250: Speed up inherited mapping assignment slot calls

[koxudaxi]

__setitem__ and __delitem__ share the mp_ass_subscript slot. If a Python subclass of a C type overrides only one of them, the inherited operation is still dispatched through slot_mp_ass_subscript() and called through a wrapper_descriptor.

When lookup returns the expected unbound wrapper descriptor, slot_mp_ass_subscript() now calls the wrapped C slot directly. Python overrides and other descriptors continue through the generic call path.

Benchmark

Release builds on macOS 15.7.1 arm64, based on main at b062727097e997bcb900e11503d3248daac903da. Results are medians from nine alternating-order samples after two warmups.

	Median	Range
main	1433.26 ms	1420.77–1500.78 ms
this PR	883.81 ms	865.64–934.20 ms

This reduces the collections.Counter assignment benchmark by 549.45 ms (38.34%).

Tests

Tests cover asymmetric dict subclasses, inherited slice deletion on a list subclass, and fallback for custom or incompatible descriptors. The targeted tests pass on regular and free-threaded debug builds.

Fixes #156250

• Issue: Improve performance of inherited mapping assignment slot calls #156250