feature: add ci testing on centos-7 platform

[ssbarnea]

As I recently faced two bugs related to the older git version that comes with centos-7, I would like to propose to add a CI test that runs pre-commit on centos-7, so we can avoid future regressions.

This testing can be implemented using a container, maybe even using tox-docker and should include at least two tests that we already know to fail:

• cloning a gitlab repo without .git at the end of url - flake8
• cloning a repo that use a commit revision instead of a tag/branch name

Is this ok? If so, what path would you recommend? I can easily make a PR to enable that.

[asottile]

no thanks, centos 7 is very old