Bump attrs from 21.2.0 to 21.4.0 (#655)

Bumps [attrs](https://github.com/python-attrs/attrs) from 21.2.0 to 21.4.0.
- [Release notes](https://github.com/python-attrs/attrs/releases)
- [Changelog](https://github.com/python-attrs/attrs/blob/main/CHANGELOG.rst)
- [Commits](python-attrs/attrs@21.2.0...21.4.0)

---
updated-dependencies:
- dependency-name: attrs
  dependency-type: direct:development
  update-type: version-update:semver-minor
...

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Co-authored-by: dependabot[bot] <49699333+dependabot[bot]@users.noreply.github.com>

Author: dependabot[bot]

requirements-dev.txt

@@ -2,7 +2,7 @@
 aiohttp==3.8.1
 aiomcache==0.6.0
 aioredis==2.0.0
-attrs==21.2.0
+attrs==21.4.0
 chardet==4.0.0
 cryptography==36.0.1
 docker==5.0.3