Fix type annotation of ast.NodeTransformer.generic_visit(). It always returns an ast.ASt object. (#4483)

stdlib/3/ast.pyi

@@ -144,7 +144,7 @@ class NodeVisitor:
     def visit_Ellipsis(self, node: Ellipsis) -> Any: ...
 
 class NodeTransformer(NodeVisitor):
-    def generic_visit(self, node: AST) -> Optional[AST]: ...
+    def generic_visit(self, node: AST) -> AST: ...
     # TODO: Override the visit_* methods with better return types.
     #       The usual return type is Optional[AST], but Iterable[AST]
     #       is also allowed in some cases -- this needs to be mapped.