Bump attrs from 21.2.0 to 21.4.0

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: indirect
  update-type: version-update:semver-minor
...

Signed-off-by: dependabot[bot] <[email protected]>

Author: dependabot[bot]

test-requirements.txt

@@ -8,7 +8,7 @@ alabaster==0.7.12
     # via sphinx
 async-generator==1.10
     # via -r test-requirements.in
-attrs==21.2.0
+attrs==21.4.0
     # via pytest
 babel==2.9.0
     # via sphinx