Registries are a core part of the extensibility in Djblets and in

Review Board, and perform registration management and fetching for

specific types of items.
To aid in type safety, this change adds typing to djblets.registries.

Registry itself is now a Generic[], allowing a subclass to specify

exactly what type of item is being stored. This then enables type safety

within the registry and for callers registering, fetching, or iterating

through items.
This unveiled an issue where ExceptionFreeGetterMixin doesn't play

especially well with Python typing, as it overrides the Registry.get()

method to use Optional[T] for the result. Type checkers don't like it

when the function signature gets replaced when mixing in or subclassing,

since it's perceived to violate the contract of the function and produce

a type-unsafe result.
We've worked around that here, but we have had situations in the past

where we've incorrectly called get() expecting None results for

invalid items when ItemLookupError would be raised instead, and

vice-versa.
To address these issues, a new Registry.get_or_none() method has been

added, which works like the mixin's get() method. Callers can move

over to that, and get() can be made deprecated by subclasses. In time,

we'll probably want to deprecate ExceptionFreeGetterMixin altogether,

but that probably won't happen until Djblets 4 at the earliest.
Some unit tests have also been fixed. We had some bad asserts

(assertTrue() instead of assertEqual()), some type-unsafe checks,

and just some very old assertions that needed updating. The tests also

now largely test typed registries.

All unit tests passed.
Verified there are no typing warnings/errors within djblets.registries

(using mypy and pyright), aside from untyped modules being imported.
Built the docs and checked for errors.