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: direct:production
  update-type: version-update:semver-minor
...

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

requirements.txt

@@ -4,7 +4,7 @@
 #
 #    pip-compile requirements.in
 #
-attrs==21.2.0
+attrs==21.4.0
     # via -r requirements.in
 marshmallow==3.14.1
     # via -r requirements.in