requirements.txt: add black

This way, IDEs can be set up to use the black version from the environment instead of a globally
available/bundled black version.

requirements.txt

@@ -41,6 +41,7 @@ setuptools>=65.5.1 # Not a direct dependency, pinned to get a security fix
 tornado==6.3.3 # Not a direct dependency, pinned to get a security fix
 
 # Dev
+black==22.*
 celery-types==0.18.0
 django-stubs[compatible-mypy]==4.2.4
 mypy==1.5.1