fix: prefer black formatter's whitespace style

Author: mihow

setup.cfg

@@ -4,6 +4,8 @@
 [flake8]
 max-line-length = 119
 exclude = .tox,.git,*/migrations/*,*/static/CACHE/*,docs,node_modules,venv,.venv
+# E203: whitespace before ':' (conflicts with Black's slice formatting)
+extend-ignore = E203, W503
 
 [pycodestyle]
 max-line-length = 119