Add special support for @django.cached_property needed in django-stubs

[sobolevn]

Hi!

We, in django-stubs, have a lot of usages of @cached_property decorator that is a part of django: https://docs.djangoproject.com/en/5.2/ref/utils/#django.utils.functional.cached_property

All usages of it we have to add to subtest/allowlist.txt, which is not great. In typing we reuse @functools.cached_property to have all the benefits of its inference: https://github.com/typeddjango/django-stubs/blob/ee8e8b11c37866969ff0406be20591a067dfa983/django-stubs/utils/functional.pyi#L3-L4

But, stubtest is not happy with this move: because in runtime objects have django.utils.functional.cached_property type and we see the following error:

error: django.http.response.HttpResponse.text is inconsistent, cannot reconcile @property on stub with runtime object
Stub: in file /home/runner/work/django-stubs/django-stubs/django-stubs/http/response.pyi:106
def (self: django.http.response.HttpResponse) -> builtins.str
Runtime:
<django.utils.functional.cached_property object at 0x7f7ce7704920>

So, we add all @django.utils.functional.cached_property usages to our allowlist.txt. There are LOTS of entries there: https://github.com/typeddjango/django-stubs/blob/ee8e8b11c37866969ff0406be20591a067dfa983/scripts/stubtest/allowlist.txt#L158-L425

Moreover, we have to always tell about this problem to new contributors on review :(

That's why I propose to special case this as we do with other property-likes.
I've tested locally and it works perfectly. I don't want to complicate the CI with django installation and special tests. So, I added # pragma: no cover to indicate that it is not tested.

[AlexWaygood]

this method is called _verify_readonly_property, but inspect.isdatadescriptor() only returns True if an object has a __set__ method (and descriptors with __set__ methods are analogous to read/write properties rather than read-only properties). Could we avoid adding special-casing for django here if we just checked whether runtime is an object with a __get__ method rather than checking if the object is a data descriptor?

Something like this? (Though you probably want to use getattr_static or something since the getattr() call here might raise TypeError or something crazy)

Suggested change

	if _is_django_cached_property(runtime):
	yield from _verify_final_method(stub.func, runtime.func, MISSING)
	return
	if inspect.isdatadescriptor(runtime):
	# It's enough like a property...
	return
	if callable(getattr(type(runtime), "__get__", None)):
	# It's enough like a property...
	return

[sobolevn]

This way I can't verify that stub.func and runtime.func are the same :(
But, this extra check can be added on its own in a separate PR.