Complete singledispatch stub (#9201)

stubs/singledispatch/@tests/stubtest_allowlist.txt

@@ -1 +1,2 @@
-singledispatch.singledispatchmethod.__call__  # A lie to reflect that the descriptor get returns a callable
+# Internal utils, we are not interested in them:
+singledispatch.helpers

stubs/singledispatch/METADATA.toml

@@ -1 +1,4 @@
 version = "3.7.*"
+
+[tool.stubtest]
+ignore_missing_stub = false

stubs/singledispatch/singledispatch.pyi

@@ -2,6 +2,7 @@ from collections.abc import Callable, Mapping
 from typing import Any, Generic, TypeVar, overload
 
 _T = TypeVar("_T")
+_S = TypeVar("_S")
 
 class _SingleDispatchCallable(Generic[_T]):
     registry: Mapping[Any, Callable[..., _T]]
@@ -19,10 +20,12 @@ class singledispatchmethod(Generic[_T]):
     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
     def register(self, cls: Callable[..., _T], method: None = ...) -> Callable[..., _T]: ...
     @overload
     def register(self, cls: type[Any], method: Callable[..., _T]) -> Callable[..., _T]: ...
-    def __call__(self, *args: Any, **kwargs: Any) -> _T: ...
+    def __get__(self, obj: _S, cls: type[_S] | None = ...) -> Callable[..., _T]: ...