Add comments when subclassing Any (#9732)

stdlib/unittest/mock.pyi

@@ -101,6 +101,8 @@ class _CallList(list[_Call]):
 class Base:
     def __init__(self, *args: Any, **kwargs: Any) -> None: ...
 
+# We subclass with "Any" because mocks are explicitly designed to stand in for other types,
+# something that can't be expressed with our static type system.
 class NonCallableMock(Base, Any):
     def __new__(__cls, *args: Any, **kw: Any) -> Self: ...
     def __init__(