Much of the code in djblets.secrets is type-specific, particularly the

crypto-related code. It's important to get this right, and there's new

in-progress code being developed that uses these methods.
To ensure everything works smoothly, this change adds type annotations

to all of djblets.secrets. This includes the crypto method and all the

token generator code.
The token generator registry now uses the new typed registries support,

ensuring type safety for all items in the registry. Since this registry

uses ExceptionFreeGetterMixin, the get() method will continue to

allow None results, at the expense of type safety warnings where the

overridden get() doesn't match the original function signature.

Ideally, we'd remove this mixin, but we'd need to go through a

deprecation cycle for the old behavior, and it isn't worth it right now.

Internally, Registry.get_or_none() is now used.

Unit tests pass for all supported versions of Python.
mypy and pyright are largely happy, with the exception of the overridden

Registry.get() method.