Bump gunicorn from 20.1.0 to 22.0.0 (#3802)

Bumps [gunicorn](https://github.com/benoitc/gunicorn) from 20.1.0 to 22.0.0.
- [Release notes](https://github.com/benoitc/gunicorn/releases)
- [Commits](https://github.com/benoitc/gunicorn/compare/20.1.0...22.0.0)

---
updated-dependencies:
- dependency-name: gunicorn
  dependency-type: direct:development
...

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requirements.dev.txt

@@ -4,7 +4,7 @@
 # provides its own copy.
 
 coverage==5.5
-gunicorn==20.1.0
+gunicorn==22.0.0
 
 # Mypy and dependencies
 mypy==1.4.1