Make SupportsConcat and variants protocols

Author: George-Ogden

stdlib/_operator.pyi

@@ -66,10 +66,10 @@ class _SupportsRAnd(Protocol[_T_contra, _U_co]):
     def __rand__(self, other: _T_contra, /) -> _U_co: ...
 
 @type_check_only
-class _SupportsConcat(SupportsGetItem[Any, Any], _SupportsAdd[_T_contra, _U_co]): ...
+class _SupportsConcat(SupportsGetItem[Any, Any], _SupportsAdd[_T_contra, _U_co], Protocol[_T_contra, _U_co]): ...
 
 @type_check_only
-class _SupportsRConcat(SupportsGetItem[Any, Any], _SupportsRAdd[_T_contra, _U_co]): ...
+class _SupportsRConcat(SupportsGetItem[Any, Any], _SupportsRAdd[_T_contra, _U_co], Protocol[_T_contra, _U_co]): ...
 
 @type_check_only
 class _SupportsFloorDiv(Protocol[_T_contra, _U_co]):
@@ -172,7 +172,7 @@ class _SupportsIAnd(Protocol[_T_contra, _U_co]):
     def __iand__(self, other: _T_contra, /) -> _U_co: ...  # noqa: Y034
 
 @type_check_only
-class _SupportsIConcat(SupportsGetItem[Any, Any], _SupportsIAdd[_T_contra, _U_co]): ...
+class _SupportsIConcat(SupportsGetItem[Any, Any], _SupportsIAdd[_T_contra, _U_co], Protocol[_T_contra, _U_co]): ...
 
 @type_check_only
 class _SupportsIFloorDiv(Protocol[_T_contra, _U_co]):