- update to 21.4.0:
* Fixed the test suite on PyPy3.8 where ``cloudpickle`` does not work.
* Fixed ``coverage report`` for projects that use ``attrs`` and don't
set a ``--source``.
* When using ``@define``, converters are now run by default when
setting an attribute on an instance -- additionally to validators.
* ``import attrs`` has finally landed!
* ``attr.asdict(retain_collection_types=False)`` (default)
dumps collection-esque keys as tuples.
* ``__match_args__`` are now generated to support Python 3.10's
* If the class-level *on_setattr* is set to ``attrs.setters.validate``
(default in ``@define`` and ``@mutable``) but no field defines
a validator, pretend that it's not set.
* The generated ``__repr__`` is significantly faster on Pythons with
f-strings.
* Attributes transformed via ``field_transformer`` are wrapped with
``AttrsClass`` again.
* Generated source code is now cached more efficiently for identical classes.
* Added ``attrs.converters.to_bool()``.
* ``attrs.resolve_types()`` now resolves types of subclasses after
the parents are resolved.
* Added new validators: ``lt(val)`` (< val), ``le(va)`` (≤ val),
``ge(val)`` (≥ val), ``gt(val)`` (> val), and ``maxlen(n)``.
* ``attrs`` classes are now fully compatible with cloudpickle
* Added new context manager ``attrs.validators.disabled()`` and
functions ``attrs.validators.(set|get)_disabled()``.
They deprecate ``attrs.(set|get)_run_validators()``.
All functions are interoperable and modify the same internal state.
They are not – and never were – thread-safe, though.