Fix test after syncing NotImplemented fixture definition with typeshed

Test case testReturnAnyForNotImplementedInNormalMethods become silently
invalid after typeshed changes it defintiion of NotImplemented from
Any to a subclass to Any.

We can store the old behavior by special casing NotImplemented in
check_return_stmt.

Author: brianschubert

mypy/checker.py

@@ -4581,6 +4581,13 @@ def check_return_stmt(self, s: ReturnStmt) -> None:
                         s.expr, return_type, allow_none_return=allow_none_func_call
                     )
                 )
+                # Treat NotImplemented as having type Any, consistent with its
+                # definition in typeshed prior to python/typeshed#4222.
+                if (
+                    isinstance(typ, Instance)
+                    and typ.type.fullname == "builtins._NotImplementedType"
+                ):
+                    typ = AnyType(TypeOfAny.special_form)
 
                 if defn.is_async_generator:
                     self.fail(message_registry.RETURN_IN_ASYNC_GENERATOR, s)