Use new official repository for flake8 on CI

.pre-commit-config.yaml

@@ -3,7 +3,7 @@ repos:
     rev: v5.10.1
     hooks:
       - id: isort
-  - repo: https://gitlab.com/pycqa/flake8
+  - repo: https://github.com/pycqa/flake8
     rev: 3.9.2
     hooks:
       - id: flake8