Update types.pyi for Python3.12

[sobolevn]

Source: https://github.com/python/cpython/blob/3.12/Objects/genobject.c#L1551

It is read-only:

>>>asyncdefsome(): ... yield1 ... >>>some() <async_generatorobjectsomeat0x10559d300>>>>some().ag_suspendedFalse>>>some().ag_suspended=TrueTraceback (mostrecentcalllast): File"<stdin>", line1, in<module>AttributeError: attribute'ag_suspended'of'async_generator'objectsisnotwritable

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉