Implement PEP 803 -- abi3t

[encukou]

PEP-803 is accepted; this tracks the implementation.

• The Py_TARGET_ABI3T knob (and interplay with Py_LIMITED_API & Py_GIL_DISABLED): gh-146636: PEP 803: add Py_TARGET_ABI3T and .abi3t.so extension #146637
• Opaque PyObject: gh-111506: Add _Py_OPAQUE_PYOBJECT to hide PyObject layout & related API #136505
• Make Py_mod_abi mandatory: gh-146636: Py_mod_abi mandatory for modules created from slots array #146855
• Guard against GIL ABI on FT: gh-137956: Guard against non-free-threaded extensions in free-threaded builds #137957
• importlib.machinery.EXTENSION_SUFFIXES changes: gh-146636: PEP 803: add Py_TARGET_ABI3T and .abi3t.so extension #146637
• Docs
• Extended tests

Linked PRs

• gh-146636: PEP 803: add Py_TARGET_ABI3T and .abi3t.so extension #146637
• gh-146636: Py_mod_abi mandatory for modules created from slots array #146855

[neutrinoceros]

@encukou congratz ! Although I think there's a typo in the title and description; you mean PEP 803, right ?