Fix false positive with calling .register() on KeysView subclass (#9348)

* Revert "`Collection` is `Sized` (#8977)"

This reverts commit 5bbba5d008.

* Revert "typing: remove metaclass from Sized (#9058)"

This reverts commit a3ce512095.

* Add regression test for issue 9296.

stdlib/typing.pyi

@@ -325,7 +325,7 @@ class SupportsRound(Protocol[_T_co]):
     def __round__(self, __ndigits: int) -> _T_co: ...
 
 @runtime_checkable
-class Sized(Protocol):
+class Sized(Protocol, metaclass=ABCMeta):
     @abstractmethod
     def __len__(self) -> int: ...
 
@@ -452,7 +452,10 @@ class Container(Protocol[_T_co]):
     def __contains__(self, __x: object) -> bool: ...
 
 @runtime_checkable
-class Collection(Sized, Iterable[_T_co], Container[_T_co], Protocol[_T_co]): ...
+class Collection(Iterable[_T_co], Container[_T_co], Protocol[_T_co]):
+    # Implement Sized (but don't have it as a base class).
+    @abstractmethod
+    def __len__(self) -> int: ...
 
 class Sequence(Collection[_T_co], Reversible[_T_co], Generic[_T_co]):
     @overload

test_cases/stdlib/typing/check_regression_issue_9296.py

@@ -0,0 +1,16 @@
+from __future__ import annotations
+
+import typing as t
+
+KT = t.TypeVar("KT")
+
+
+class MyKeysView(t.KeysView[KT]):
+    pass
+
+
+d: dict[t.Any, t.Any] = {}
+dict_keys = type(d.keys())
+
+# This should not cause an error like `Member "register" is unknown`:
+MyKeysView.register(dict_keys)