moving flake8 from gitlab to github;

[vincentpierre]

Motivation and Context

https://gitlab.com/pycqa/flake8 now seem to require authentication, which makes our pre-commit CI fail. In this PR, the source repo was changed to https://github.com/PyCQA/flake8

How Has This Been Tested

pre-commit still performs flake8 check on local pre-commit

Types of changes

• Docs change / refactoring / dependency upgrade
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have completed my CLA (see CONTRIBUTING)
• I have added tests to cover my changes.
• All new and existing tests passed.

[Skylion007]

If you are curious what happened: https://www.youtube.com/watch?v=1FIgj9Q5e6A