typing: make Generic definition reflect runtime (#14583)

Co-authored-by: Alex Waygood <Alex.Waygood@Gmail.com>

stdlib/@tests/stubtest_allowlists/common.txt

@@ -466,7 +466,6 @@ typing._Final.__init_subclass__
 typing\.Protocol
 typing(_extensions)?\._TypedDict
 typing(_extensions)?\.Any.*
-typing(_extensions)?\.Generic
 typing(_extensions)?\.TypedDict
 typing_extensions\.ParamSpec.*
 typing_extensions\.TypeVar.*

stdlib/typing.pyi

@@ -235,7 +235,6 @@ class _SpecialForm(_Final):
         def __ror__(self, other: Any) -> _SpecialForm: ...
 
 Union: _SpecialForm
-Generic: _SpecialForm
 Protocol: _SpecialForm
 Callable: _SpecialForm
 Type: _SpecialForm
@@ -440,6 +439,20 @@ Annotated: _SpecialForm
 # Predefined type variables.
 AnyStr = TypeVar("AnyStr", str, bytes)  # noqa: Y001
 
+@type_check_only
+class _Generic:
+    if sys.version_info < (3, 12):
+        __slots__ = ()
+
+    if sys.version_info >= (3, 10):
+        @classmethod
+        def __class_getitem__(cls, args: TypeVar | ParamSpec | tuple[TypeVar | ParamSpec, ...]) -> _Final: ...
+    else:
+        @classmethod
+        def __class_getitem__(cls, args: TypeVar | tuple[TypeVar, ...]) -> _Final: ...
+
+Generic: type[_Generic]
+
 class _ProtocolMeta(ABCMeta):
     if sys.version_info >= (3, 12):
         def __init__(cls, *args: Any, **kwargs: Any) -> None: ...