Change type read-only attributes from @property to Final

[crypt0lith]

Follow-up to #16549

[github-actions]

Diff from mypy_primer, showing the effect of this PR on open source code:

scipy-stubs (https://github.com/scipy/scipy-stubs)
+ tests/misc/test_submodules_empty.pyi:3: note: See https://mypy.readthedocs.io/en/stable/running_mypy.html#missing-imports

[JelleZijlstra]

Final also means they can't be overridden in subclasses. Is that accurate for these attributes?

[crypt0lith]

Python does not prohibit them from being overridden, but doing so causes Python to disagree with the C implementation:

>>> class Meta(type):
...     __base__ = str
...     __basicsize__ = \
...     __dictoffset__ = \
...     __flags__ = \
...     __itemsize__ = \
...     __weakrefoffset__ = 42
...     __text_signature__ = "hello"
...     
>>> class Foo(metaclass=Meta):
...     pass
...     
>>> print(
...     *(
...         (getattr(Foo, attr), type.__dict__[attr].__get__(Foo, Meta))
...         for attr in [
...             "__base__",
...             "__basicsize__",
...             "__dictoffset__",
...             "__flags__",
...             "__itemsize__",
...             "__weakrefoffset__",
...             "__text_signature__",
...         ]
...     ),
...     sep="\n",
... )
(<class 'str'>, <class 'object'>)
(42, 16)
(42, -1)
(42, 22044)
(42, 0)
(42, -32)
('hello', None)

So Final here is explicitly declaring that those attributes should not be overridden.

inspect.isabstract accesses __flags__, so that is one example where allowing an override would be unsafe.