Make verify first type a supertype of all dispatch types (#10727)

Change the type of the first argument of verify (the type that
singledispatch is dispatching on) to be a supertype of the dispatch
types of all registered implementations, including the implementation
for Missing.

mypy/stubtest.py

@@ -172,7 +172,7 @@ def test_module(module_name: str) -> Iterator[Error]:
 
 @singledispatch
 def verify(
-    stub: nodes.Node, runtime: MaybeMissing[Any], object_path: List[str]
+    stub: MaybeMissing[nodes.Node], runtime: MaybeMissing[Any], object_path: List[str]
 ) -> Iterator[Error]:
     """Entry point for comparing a stub to a runtime object.