pip(deps): update attrs requirement from ~=21.2 to ~=21.4

[dependabot]

Updates the requirements on attrs to permit the latest version.

Release notes

Sourced from attrs's releases.

21.4.0

This is a quick bug fix release for a regression that causes coverage report fail under certain circumstances.

Full Changelog

Changes

• Fixed the test suite on PyPy3.8 where cloudpickle does not work. #892
• Fixed coverage report for projects that use attrs and don't set a --source. #895, #896

Changelog

Sourced from attrs's changelog.

21.4.0 (2021-12-29)

Changes ^^^^^^^

• Fixed the test suite on PyPy3.8 where cloudpickle does not work. [#892](https://github.com/python-attrs/attrs/issues/892) <https://github.com/python-attrs/attrs/issues/892>_
• Fixed coverage report for projects that use attrs and don't set a --source. [#895](https://github.com/python-attrs/attrs/issues/895) <https://github.com/python-attrs/attrs/issues/895>, [#896](https://github.com/python-attrs/attrs/issues/896) <https://github.com/python-attrs/attrs/issues/896>

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21.3.0 (2021-12-28)

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

• When using @define, converters are now run by default when setting an attribute on an instance -- additionally to validators. I.e. the new default is on_setattr=[attrs.setters.convert, attrs.setters.validate].

  This is unfortunately a breaking change, but it was an oversight, impossible to raise a DeprecationWarning about, and it's better to fix it now while the APIs are very fresh with few users. [#835](https://github.com/python-attrs/attrs/issues/835) <https://github.com/python-attrs/attrs/issues/835>, [#886](https://github.com/python-attrs/attrs/issues/886) <https://github.com/python-attrs/attrs/issues/886>

• import attrs has finally landed! As of this release, you can finally import attrs using its proper name.

  Not all names from the attr namespace have been transferred; most notably attr.s and attr.ib are missing. See attrs.define and attrs.field if you haven't seen our next-generation APIs yet. A more elaborate explanation can be found On The Core API Names <https://www.attrs.org/en/latest/names.html>_

  This feature is at least for one release provisional. We don't plan on changing anything, but such a big change is unlikely to go perfectly on the first strike.

  The API docs have been mostly updated, but it will be an ongoing effort to change everything to the new APIs. Please note that we have not moved -- or even removed -- anything from attr!

  Please do report any bugs or documentation inconsistencies! [#887](https://github.com/python-attrs/attrs/issues/887) <https://github.com/python-attrs/attrs/issues/887>_

Changes ^^^^^^^

• attr.asdict(retain_collection_types=False) (default) dumps collection-esque keys as tuples. [#646](https://github.com/python-attrs/attrs/issues/646) <https://github.com/python-attrs/attrs/issues/646>_,

... (truncated)

Commits

• 2de9014 Prepare 21.4.0
• 02ba249 Remove dead anchors
• 1ff3f1e Minor changelog polish
• 0575d51 Make virtual repr file names unique (#896)
• 03dd713 Do not require cloudpickle for PyPy (#892)
• ce50f40 Use correct words
• 9424a93 Tell coverage about import attrs
• c86fbc8 Use better word
• 7695908 docs: fix very minor typo (#894)
• ada66be docs: remove typo in README.rst (#893)
• Additional commits viewable in compare view

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[coveralls]

Pull Request Test Coverage Report for Build 1841405775

• 0 of 0 changed or added relevant lines in 0 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.2%) to 96.158%

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Totals
Change from base Build 1807569458:	0.2%
Covered Lines:	1777
Relevant Lines:	1848

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💛 - Coveralls

[ncilfone]

@dependabot rebase