AppliesToURLMixin had the wrong base class, using Extension instead

of ExtensionHook, which ended up breaking the typing for

initialize() and other methods for anything subclassing a hook using

AppliesToURLMixin.
This fixes that, switches to Sequence[str] instead of List[str] for

the parameter, and adds typing for the attribute.

Unit tests pass.
Verified this fixed issues I hit with some extension work.