bpo-46246: add missing __slots__ to importlib.metadata.DeprecatedList (GH-30452)

Confirmed with @jaraco that this indeed needs a fix.

A question that came up while I was digging into the code: I think `SelectableGroups` could similarly use `__slots__ = ()`, since its purpose seems only for convenience around `dict`, not to have attributes of its own.

Automerge-Triggered-By: GH:jaraco