Implement PEP 820 – PySlot: Unified slot system for the C API

[encukou]

Feature or enhancement

PEP-820 was accepted, I'll send a PR soon.

Linked PRs

• gh-149044: Implement PEP 820 – PySlot: Unified slot system for the C API #149055
• gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases #151252
• [3.15] gh-149044: Improve Py_tp_base[s] docs & error message for non-type bases (GH-151252) #151606
• gh-149044: Fix PySlot_END macro for C++ #158866
• gh-149044: Build test_cext with -Wall -Wextra #158867
• [3.15] gh-149044: Avoid C++ warnings in PySlot_END macro (GH-158866) #158965