Jsonschema

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* Warn at runtime when subclassing validator classes. Doing so was not
intended to be public API, though it seems some downstream libraries
do so. A future version will make this an error, as it is brittle and
better served by composing validator objects instead. Feel free to reach
out if there are any cases where changing existing code seems difficult
and I can try to provide guidance.

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* Make the rendered README in PyPI simpler and fancier. Thanks Hynek (983)!

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* ``jsonschema.validators.validator_for`` now properly uses the explicitly
provided default validator even if the ``$schema`` URI is not found.

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* Fix a second place where subclasses may have added attrs attributes (982).

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* Fix Validator.evolve (and APIs like ``iter_errors`` which call it) for cases
where the validator class has been subclassed. Doing so wasn't intended to be
public API, but given it didn't warn or raise an error it's of course
understandable. The next release however will make it warn (and a future one
will make it error). If you need help migrating usage of inheriting from a
validator class feel free to open a discussion and I'll try to give some
guidance (982).

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* Add support for referencing schemas with ``$ref`` across different versions
of the specification than the referrer's