[IMP] Change pre-commit flake8 URL

https://gitlab.com/pycqa/flake8 is currently blocked
for some countries and this gitlab repository is a mirror of
the github one https://github.com/pycqa/flake8

.pre-commit-config.yaml

@@ -25,7 +25,7 @@ repos:
       - id: mixed-line-ending
         args: ["--fix=lf"]
       - id: trailing-whitespace
-  - repo: https://gitlab.com/pycqa/flake8
+  - repo: https://github.com/pycqa/flake8
     rev: 3.9.2
     hooks:
       - id: flake8