Improve tuple.__add__ type (#5522)

Noticed this while browsing the stub. This feels safe but we'll see what mypy-primer has to say.

Author: JelleZijlstra

stdlib/builtins.pyi

@@ -724,7 +724,7 @@ class tuple(Sequence[_T_co], Generic[_T_co]):
     @overload
     def __add__(self, x: Tuple[_T_co, ...]) -> Tuple[_T_co, ...]: ...
     @overload
-    def __add__(self, x: Tuple[Any, ...]) -> Tuple[Any, ...]: ...
+    def __add__(self, x: Tuple[_T, ...]) -> Tuple[Union[_T_co, _T], ...]: ...
     def __mul__(self, n: int) -> Tuple[_T_co, ...]: ...
     def __rmul__(self, n: int) -> Tuple[_T_co, ...]: ...
     def count(self, __value: Any) -> int: ...