bpo-36972: Add SupportsIndex (GH-13448)

In order to support typing checks calling hex(), oct() and bin() on user-defined classes, a SupportIndex protocol is required. The ability to check these at runtime would be good to add for completeness sake. This is pretty much just a copy of SupportsInt with the names tweaked.

Lib/typing.py

@@ -74,6 +74,7 @@ __all__ = [
     'SupportsBytes',
     'SupportsComplex',
     'SupportsFloat',
+    'SupportsIndex',
     'SupportsInt',
     'SupportsRound',
 
@@ -1304,6 +1305,14 @@ class SupportsBytes(_Protocol):
         pass
 
 
+class SupportsIndex(_Protocol):
+    __slots__ = ()
+
+    @abstractmethod
+    def __index__(self) -> int:
+        pass
+
+
 class SupportsAbs(_Protocol[T_co]):
     __slots__ = ()