Add black to dev dependencies

So Django's auto formatter for generated files will pick it up

> Python files created by startproject, startapp, optimizemigration, makemigrations, and squashmigrations are now formatted using the black command if it is present on your PATH.

https://docs.djangoproject.com/en/5.2/releases/4.1/

Author: ulgens

requirements/dev.txt

@@ -1,4 +1,5 @@
 -r common.txt
+black==25.1.0  # Used by Used by Django's find_formatters and run_formatters
 django-debug-toolbar==5.2.0
 pre-commit~=4.2.0
 watchdog==6.0.0