Similar to #11817.
In `_create_power_level_validator` we
- retrieve `validator`. This is a class implementing the
`jsonschema.protocols.Validator` interface. In other words,
`validator: Type[jsonschema.protocols.Validator]`.
- we then create an second validator class by modifying the original
`validator`. We return that class, which is also of type
`Type[jsonschema.protocols.Validator]`.
So the original annotation was incorrect: it claimed we were returning
an instance of jsonSchema.Draft7Validator, not the class (or a subclass)
itself. (Strictly speaking this is incorrect, because `POWER_LEVELS_SCHEMA`
isn't pinned to a particular version of JSON Schema. But there are other
complications with the type stubs if you try to fix this; I felt like
the change herein was a decent compromise that better expresses intent).
(I suspect/hope the typeshed project would welcome an effort to improve
the jsonschema stubs. Let's see if I get some spare time.)
