Add missing int.__index__() signature (#2335)

This has been present since Python 2.5.

stdlib/2/__builtin__.pyi

@@ -139,6 +139,7 @@ class int:
     def __abs__(self) -> int: ...
     def __hash__(self) -> int: ...
     def __nonzero__(self) -> bool: ...
+    def __index__(self) -> int: ...
 
 class float:
     def __init__(self, x: Union[SupportsFloat, str, unicode, bytearray] = ...) -> None: ...

stdlib/2/builtins.pyi

@@ -139,6 +139,7 @@ class int:
     def __abs__(self) -> int: ...
     def __hash__(self) -> int: ...
     def __nonzero__(self) -> bool: ...
+    def __index__(self) -> int: ...
 
 class float:
     def __init__(self, x: Union[SupportsFloat, str, unicode, bytearray] = ...) -> None: ...

stdlib/3/builtins.pyi

@@ -158,6 +158,7 @@ class int:
     def __abs__(self) -> int: ...
     def __hash__(self) -> int: ...
     def __bool__(self) -> bool: ...
+    def __index__(self) -> int: ...
 
 class float:
     def __init__(self, x: Union[SupportsFloat, str, bytes] = ...) -> None: ...