build(deps): bump attrs from 21.3.0 to 21.4.0

Bumps [attrs](https://github.com/python-attrs/attrs) from 21.3.0 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@21.3.0...21.4.0)

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

Signed-off-by: dependabot[bot] <support@github.com>

requirements.txt

@@ -1,7 +1,7 @@
 appdirs==1.4.4
 astroid==2.9.0
 atomicwrites==1.4.0
-attrs==21.3.0
+attrs==21.4.0
 black==21.4b2
 certifi==2021.10.8
 cfgv==3.3.1