Update flake8 url in pre-commit

[noelchen90]

Today's pre-commits were effected by this error message:

stderr:
    remote: The project you were looking for could not be found or you don't have permission to view it.
    fatal: repository '<https://gitlab.com/pycqa/flake8.git/>' not found

This is because flake8 migrated from gitlab to github (see PyCQA's own PR)
The new link is now: https://github.com/PyCQA/flake8.git

Pre-commits should be fine after this update.

[lyuyangh]

Thanks!