The new djblets.util.typing module includes some standard types for

JSON data, keyword argument dictionaries, and safe aliases for

django-stubs StrOrPromise implementations.
The new JSON types are enhanced versions of our old ones. Modern

versions of mypy and pyright offer recursive type checks, meaning we can

now have deep JSON structures that still type correctly.
The StrOrPromise/StrPromise types alias the ones in django-stubs

when type checking, and otherwise fall back to an approximation. This

mostly mirrors what the django_stubs_ext module gives us (which they

recommend for type checks), but then we'd have to depend on that at

runtime. Plus, this a bit of a better/more correctly typed

implementation.
The new djblets.util.symbols module gives us common symbols we can use

elsewhere. For now, this provides an UNSET symbol, useful as default

arguments for other functions where it's important to differentiate from

unset and None.
Other modules have been updated to use some of these where appropriate.

Not all of this is currently in use within Djblets, but future changes

will build off of these.

Unit tests pass for all supported versions of Python.
Made use of these in some upcoming changes.
Built some test code to ensure that the types were being interpreted

correctly, in particular the recursive JSON typing.