Jsonschema

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* Make ``ErrorTree`` have a more grammatically correct ``repr``.

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* Improve the base URI behavior when resolving a ``$ref`` to a resolution URI
which is different from the resolved schema's declared ``$id``.
* Accessing ``jsonschema.draftN_format_checker`` is deprecated. Instead, if you
want access to the format checker itself, it is exposed as
``jsonschema.validators.DraftNValidator.FORMAT_CHECKER`` on any
``jsonschema.protocols.Validator``.

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* A specific API Reference page is now present in the documentation.
* ``$ref`` on earlier drafts (specifically draft 7 and 6) has been "fixed" to
follow the specified behavior when present alongside a sibling ``$id``.
Specifically the ID is now properly ignored, and references are resolved
against whatever resolution scope was previously relevant.

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* ``FormatChecker.cls_checks`` is deprecated. Use ``FormatChecker.checks`` on
an instance of ``FormatChecker`` instead.
* ``unevaluatedItems`` has been fixed for draft 2019. It's nonetheless
discouraged to use draft 2019 for any schemas, new or old.
* Fix a number of minor annotation issues in ``protocols.Validator``

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* Add support for creating validator classes whose metaschema uses a different
dialect than its schemas. In other words, they may use draft2020-12 to define
which schemas are valid, but the schemas themselves use draft7 (or a custom
dialect, etc.) to define which *instances* are valid. Doing this is likely
not something most users, even metaschema authors, may need, but occasionally
will be useful for advanced use cases.

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* Fix some stray comments in the README.