Add missing __isabstractmethods__ attributes in abc and `functools1 (#7205)

stdlib/functools.pyi

@@ -99,6 +99,8 @@ if sys.version_info >= (3, 8):
         dispatcher: _SingleDispatchCallable[_T]
         func: Callable[..., _T]
         def __init__(self, func: Callable[..., _T]) -> None: ...
+        @property
+        def __isabstractmethod__(self) -> bool: ...
         @overload
         def register(self, cls: type[Any], method: None = ...) -> Callable[[Callable[..., _T]], Callable[..., _T]]: ...
         @overload