[stubtest] Add __warningregistry__ to the list of ignored module dunders (#14218)

We have multiple of these in `typeshed`:
https://github.com/python/typeshed/search?q=__warningregistry__

Author: sobolevn

mypy/stubtest.py

@@ -1243,6 +1243,8 @@ def verify_typealias(
         "__annotations__",
         "__path__",  # mypy adds __path__ to packages, but C packages don't have it
         "__getattr__",  # resulting behaviour might be typed explicitly
+        # Created by `warnings.warn`, does not make much sense to have in stubs:
+        "__warningregistry__",
         # TODO: remove the following from this list
         "__author__",
         "__version__",