constructors: Fix defaulted TypeVars (#7965)

From the list in https://github.com/microsoft/pyright/issues/3501

stubs/SQLAlchemy/sqlalchemy/util/_collections.pyi

@@ -91,7 +91,10 @@ else:
 def sort_dictionary(d, key: Any | None = ...): ...
 
 class OrderedSet(set[_T], Generic[_T]):
-    def __init__(self, d: Iterable[_T] | None = ...) -> None: ...
+    @overload
+    def __init__(self, d: None = ...) -> None: ...
+    @overload
+    def __init__(self, d: Iterable[_T]) -> None: ...
     def add(self, element: _T) -> None: ...
     def remove(self, element: _T) -> None: ...
     def insert(self, pos: int, element: _T) -> None: ...