Change the return type of __reduce__ and __reduce_ex__ to Union[str, Tuple[Any, ...]] (#3453)

This allows subclasses to return strings (which are allowed) from these
methods without having mypy throwing an error.

Closes #3452

Author: MSeifert04

stdlib/2/__builtin__.pyi

@@ -61,8 +61,8 @@ class object:
     def __getattribute__(self, name: str) -> Any: ...
     def __delattr__(self, name: str) -> None: ...
     def __sizeof__(self) -> int: ...
-    def __reduce__(self) -> Tuple[Any, ...]: ...
-    def __reduce_ex__(self, protocol: int) -> Tuple[Any, ...]: ...
+    def __reduce__(self) -> Union[str, Tuple[Any, ...]]: ...
+    def __reduce_ex__(self, protocol: int) -> Union[str, Tuple[Any, ...]]: ...
     if sys.version_info >= (3,):
         def __dir__(self) -> Iterable[str]: ...
     if sys.version_info >= (3, 6):

stdlib/2and3/builtins.pyi

@@ -61,8 +61,8 @@ class object:
     def __getattribute__(self, name: str) -> Any: ...
     def __delattr__(self, name: str) -> None: ...
     def __sizeof__(self) -> int: ...
-    def __reduce__(self) -> Tuple[Any, ...]: ...
-    def __reduce_ex__(self, protocol: int) -> Tuple[Any, ...]: ...
+    def __reduce__(self) -> Union[str, Tuple[Any, ...]]: ...
+    def __reduce_ex__(self, protocol: int) -> Union[str, Tuple[Any, ...]]: ...
     if sys.version_info >= (3,):
         def __dir__(self) -> Iterable[str]: ...
     if sys.version_info >= (3, 6):