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] <support@github.com>

Author: dependabot[bot]

requirements.txt

@@ -12,7 +12,7 @@ aiohttp==3.7.4.post0
     # via fdiff (setup.py)
 async-timeout==3.0.1
     # via aiohttp
-attrs==21.2.0
+attrs==21.4.0
     # via aiohttp
 cffi==1.15.0
     # via pycares