Update attrs to 21.2.0

[pyup-bot]

This PR updates attrs from 20.2.0 to 21.2.0.

Changelog

21.2.0

-------------------

Backward-incompatible Changes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

- We had to revert the recursive feature for ``attr.evolve()`` because it broke some use-cases -- sorry!
`806 <https://github.com/python-attrs/attrs/issues/806>`_
- Python 3.4 is now blocked using packaging metadata because ``attrs`` can't be imported on it anymore.
To ensure that 3.4 users can keep installing  ``attrs`` easily, we will `yank <https://pypi.org/help/#yanked>`_ 21.1.0 from PyPI.
This has **no** consequences if you pin ``attrs`` to 21.1.0.
`807 <https://github.com/python-attrs/attrs/issues/807>`_


----

21.1.0

-------------------

Deprecations
^^^^^^^^^^^^

- The long-awaited, much-talked-about, little-delivered ``import attrs`` is finally upon us!

Since the NG APIs have now been proclaimed stable, the **next** release of ``attrs`` will allow you to actually ``import attrs``.
We're taking this opportunity to replace some defaults in our APIs that made sense in 2015, but don't in 2021.

So please, if you have any pet peeves about defaults in ``attrs``'s APIs, *now* is the time to air your grievances in 487!
We're not gonna get such a chance for a second time, without breaking our backward-compatibility guarantees, or long deprecation cycles.
Therefore, speak now or forever hold you peace!
`487 <https://github.com/python-attrs/attrs/issues/487>`_
- The *cmp* argument to ``attr.s()`` and `attr.ib()` has been **undeprecated**
It will continue to be supported as syntactic sugar to set *eq* and *order* in one go.

I'm terribly sorry for the hassle around this argument!
The reason we're bringing it back is it's usefulness regarding customization of equality/ordering.

The ``cmp`` attribute and argument on ``attr.Attribute`` remains deprecated and will be removed later this year.
`773 <https://github.com/python-attrs/attrs/issues/773>`_


Changes
^^^^^^^

- It's now possible to customize the behavior of ``eq`` and ``order`` by passing in a callable.
`435 <https://github.com/python-attrs/attrs/issues/435>`_,
`627 <https://github.com/python-attrs/attrs/issues/627>`_
- The instant favorite `next-generation APIs <https://www.attrs.org/en/stable/api.html#next-gen>`_ are not provisional anymore!

They are also officially supported by Mypy as of their `0.800 release <https://mypy-lang.blogspot.com/2021/01/mypy-0800-released.html>`_.

We hope the next release will already contain an (additional) importable package called ``attrs``.
`668 <https://github.com/python-attrs/attrs/issues/668>`_,
`786 <https://github.com/python-attrs/attrs/issues/786>`_
- If an attribute defines a converter, the type of its parameter is used as type annotation for its corresponding ``__init__`` parameter.

If an ``attr.converters.pipe`` is used, the first one's is used.
`710 <https://github.com/python-attrs/attrs/issues/710>`_
- Fixed the creation of an extra slot for an ``attr.ib`` when the parent class already has a slot with the same name.
`718 <https://github.com/python-attrs/attrs/issues/718>`_
- ``__attrs__init__()`` will now be injected if ``init=False``, or if ``auto_detect=True`` and a user-defined ``__init__()`` exists.

This enables users to do "pre-init" work in their ``__init__()`` (such as ``super().__init__()``).

``__init__()`` can then delegate constructor argument processing to ``self.__attrs_init__(*args, **kwargs)``.
`731 <https://github.com/python-attrs/attrs/issues/731>`_
- ``bool(attr.NOTHING)`` is now ``False``.
`732 <https://github.com/python-attrs/attrs/issues/732>`_
- It's now possible to use ``super()`` inside of properties of slotted classes.
`747 <https://github.com/python-attrs/attrs/issues/747>`_
- Allow for a ``__attrs_pre_init__()`` method that -- if defined -- will get called at the beginning of the ``attrs``-generated ``__init__()`` method.
`750 <https://github.com/python-attrs/attrs/issues/750>`_
- Added forgotten ``attr.Attribute.evolve()`` to type stubs.
`752 <https://github.com/python-attrs/attrs/issues/752>`_
- ``attrs.evolve()`` now works recursively with nested ``attrs`` classes.
`759 <https://github.com/python-attrs/attrs/issues/759>`_
- Python 3.10 is now officially supported.
`763 <https://github.com/python-attrs/attrs/issues/763>`_
- ``attr.resolve_types()`` now takes an optional *attrib* argument to work inside a ``field_transformer``.
`774 <https://github.com/python-attrs/attrs/issues/774>`_
- ``ClassVar``\ s are now also detected if they come from `typing-extensions <https://pypi.org/project/typing-extensions/>`_.
`782 <https://github.com/python-attrs/attrs/issues/782>`_
- To make it easier to customize attribute comparison (435), we have added the ``attr.cmp_with()`` helper.

See the `new docs on comparison <https://www.attrs.org/en/stable/comparison.html>`_ for more details.
`787 <https://github.com/python-attrs/attrs/issues/787>`_
- Added **provisional** support for static typing in ``pyright`` via the `dataclass_transforms specification <https://github.com/microsoft/pyright/blob/master/specs/dataclass_transforms.md>`_.
Both the ``pyright`` specification and ``attrs`` implementation may change in future versions of both projects.

Your constructive feedback is welcome in both `attrs795 <https://github.com/python-attrs/attrs/issues/795>`_ and `pyright#1782 <https://github.com/microsoft/pyright/discussions/1782>`_.
`796 <https://github.com/python-attrs/attrs/issues/796>`_


----

20.3.0

-------------------

Backward-incompatible Changes
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

- ``attr.define()``, ``attr.frozen()``, ``attr.mutable()``, and ``attr.field()`` remain **provisional**.

This release does **not** change change anything about them and they are already used widely in production though.

If you wish to use them together with mypy, you can simply drop `this plugin <https://gist.github.com/hynek/1e3844d0c99e479e716169034b5fa963#file-attrs_ng_plugin-py>`_ into your project.

Feel free to provide feedback to them in the linked issue 668.

We will release the ``attrs`` namespace once we have the feeling that the APIs have properly settled.
`668 <https://github.com/python-attrs/attrs/issues/668>`_


Changes
^^^^^^^

- ``attr.s()`` now has a *field_transformer* hook that is called for all ``Attribute``\ s and returns a (modified or updated) list of ``Attribute`` instances.
``attr.asdict()`` has a *value_serializer* hook that can change the way values are converted.
Both hooks are meant to help with data (de-)serialization workflows.
`653 <https://github.com/python-attrs/attrs/issues/653>`_
- ``kw_only=True`` now works on Python 2.
`700 <https://github.com/python-attrs/attrs/issues/700>`_
- ``raise from`` now works on frozen classes on PyPy.
`703 <https://github.com/python-attrs/attrs/issues/703>`_,
`712 <https://github.com/python-attrs/attrs/issues/712>`_
- ``attr.asdict()`` and ``attr.astuple()`` now treat ``frozenset``\ s like ``set``\ s with regards to the *retain_collection_types* argument.
`704 <https://github.com/python-attrs/attrs/issues/704>`_
- The type stubs for ``attr.s()`` and ``attr.make_class()`` are not missing the *collect_by_mro* argument anymore.
`711 <https://github.com/python-attrs/attrs/issues/711>`_


----

Links

• PyPI: https://pypi.org/project/attrs
• Changelog: https://pyup.io/changelogs/attrs/
• Homepage: https://www.attrs.org/

[pyup-bot]

Closing this in favor of #503