Update pre-commit for flake8 move (#7907)

See: https://twitter.com/codewithanthony/status/1378746934928699396

Author: adamchainz

.pre-commit-config.yaml

@@ -12,7 +12,7 @@ repos:
   rev: 5.8.0
   hooks:
   - id: isort
-- repo: https://gitlab.com/pycqa/flake8
+- repo: https://github.com/PyCQA/flake8
   rev: 3.9.0
   hooks:
   - id: flake8