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

Author: AlexWaygood

stdlib/abc.pyi

@@ -27,7 +27,9 @@ class abstractstaticmethod(staticmethod[_R_co], Generic[_R_co]):
     __isabstractmethod__: Literal[True]
     def __init__(self, callable: Callable[..., _R_co]) -> None: ...
 
-class abstractproperty(property): ...
+class abstractproperty(property):
+    __isabstractmethod__: Literal[True]
+
 class ABC(metaclass=ABCMeta): ...
 
 def get_cache_token() -> object: ...

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