Update flake8 pre-commit link to GitHub

They moved: https://www.youtube.com/watch?v=1FIgj9Q5e6A

.pre-commit-config.yaml

@@ -13,7 +13,7 @@ repos:
     rev: 5.6.4
     hooks:
       - id: isort
-  - repo: https://gitlab.com/pycqa/flake8
+  - repo: https://github.com/pycqa/flake8
     # flake8 config is in setup.cfg
     rev: 3.8.4
     hooks: