Referencing happens to not support it at the minute.

It's close to EOL, so rather than adding it, I suspect
it's droppable.

Still will be tweaked as the referencing library's public API changes.

Internal uses of it will be removed, replaced with referencing's
resolution APIs, though RefResolver will continue to function during
its deprecation period.

This will make it easier to swap over to referencing's
Resolver (while preserving backwards compat for anyone who
passes a RefResolver to a Validator).

At some point in the future this method may become public
(which will make it easier for external dialects to resolve
references) but let's keep it private for a bit until it's
clear that the interface is stable -- a future draft might
do crazy things like have adjacent properties to $ref affect
the resolution behavior, which would make this method need to
take more than just the $ref value.

It will imminently be replaced by referencing.Registry-based
resolution.

Just avoids a deprecation warning when we switch over.

Makes way for our newer resolver to live in the shorter name.

(This should have no effect on the public API, where we have
Validator.resolver returning this attribute after emitting a deprecation
warning.)

Also bumps the minimum attrs version we depend on, as we need the alias
functionality.

Passes all the remaining referencing tests across all drafts, hooray!

Makes Validators take a referencing.Registry argument which users should
use to customize preloaded schemas, or to configure remote reference
retrieval.

This fully obsoletes jsonschema.RefResolver, which has already been
deprecated in a previous commit. Users should move to instead loading
schemas into referencing.Registry objects.

See the referencing documentation at https://referencing.rtfd.io/ for
details (with more jsonschema-specific information to be added shortly).

Note that the interface for resolving references on a Validator is not
yet public (and hidden behind _resolver and _validate_reference
attributes). One or both of these are likely to become public after some
period of stabilization.

Feedback is of course welcome!

<19.3 can have performance problems on 3.11 where there aren't
wheels (and where I think it's falling back to using the pure-
Python implementation even on CPython). Here it's ~2x slower.

Closes: #523

    * RefResolutionError is deprecated entirely.
      Use referencing.Registry-based APIs, and catch
      referencing.exceptions.Unresolvable if you really want to ignore
      referencing related issues.

    * FormatError should now be imported from jsonschema.exceptions
      only, not from the package root.

    * ErrorTree should now be imported from jsonschema.exceptions
      only, not from the package root.

Just use virtue in CI.

We're not a general class, so we know what fields we need
ahead of time.

This seems to give ~15% speedup on Validator evolution, which
happens often as part of walking up and down schemas.