[DIFF] Immutability for NoGIL for 3.15a

[xFrednet]

This is the entire diff to implement shallow and deep immutability for Python. This also includes the write-barriers from #63. The relevant files for this PR are probably:

Include/cpython/immutability.h
Include/internal/pycore_gc.h
Include/internal/pycore_immutability.h
Include/internal/pycore_object.h
Include/internal/pycore_stackref.h
Include/internal/pycore_weakref.h
Include/object.h
Include/refcount.h
Lib/immutable.py
Modules/_immutablemodule.c
Modules/_weakref.c
Objects/object.c
Objects/weakrefobject.c
Python/ceval.c
Python/crossinterp.c
Python/gc.c
Python/immutability.c

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📚 Documentation preview 📚: https://cpython-previews--112.org.readthedocs.build/

[xFrednet]

The benchmark numbers after the rewrite for GIL-enabled are in:

It is slightly worse with a Geomean slowdown of 1.042. We can probably optimize it more, but for now, this should be good enough to rebase regions on it.