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.

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)