Bump flake8-simplify from 0.14.2 to 0.21.0 in /requirements

Bumps [flake8-simplify](https://github.com/MartinThoma/flake8-simplify) from 0.14.2 to 0.21.0.
- [Release notes](https://github.com/MartinThoma/flake8-simplify/releases)
- [Changelog](https://github.com/MartinThoma/flake8-simplify/blob/main/CHANGELOG.md)
- [Commits](MartinThoma/flake8-simplify@0.14.2...0.21.0)

---
updated-dependencies:
- dependency-name: flake8-simplify
  dependency-type: direct:production
  update-type: version-update:semver-minor
...

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

requirements/lint.txt

@@ -8,7 +8,7 @@ flake8==5.0.4
 flake8-bugbear==22.9.23
 flake8-future-import==0.4.7
 flake8-no-u-prefixed-strings==0.2
-flake8-simplify==0.14.2
+flake8-simplify==0.21.0
 flake8-sfs==0.0.4
 flake8-tidy-imports==4.8.0
 mccabe==0.7.0