remove "return" statements from builtins.pyi (#986)

Type checkers shouldn't care that these methods are returning the same object.

stdlib/3/builtins.pyi

@@ -131,7 +131,7 @@ class int(SupportsInt, SupportsFloat, SupportsAbs[int]):
 
     def __str__(self) -> str: ...
     def __float__(self) -> float: ...
-    def __int__(self) -> int: return self
+    def __int__(self) -> int: ...
     def __abs__(self) -> int: ...
     def __hash__(self) -> int: ...
     def __bool__(self) -> bool: ...
@@ -282,7 +282,7 @@ class str(Sequence[str]):
     def __len__(self) -> int: ...
     def __contains__(self, s: object) -> bool: ...
     def __iter__(self) -> Iterator[str]: ...
-    def __str__(self) -> str: return self
+    def __str__(self) -> str: ...
     def __repr__(self) -> str: ...
     def __int__(self) -> int: ...
     def __float__(self) -> float: ...