Add GitLab fetcher

[suhanoves]

Proposed changes

Hello Augusto, Thank you for developing such a fantastic library!

I've been working on incorporating the ability to store style files on a self-hosted GitLab instance, which I noticed was missing from the library, so I took the initiative to add this feature.

I've made an effort to thoroughly document the nuances of interfacing with GitLab's API in the method docstrings. Unlike GitHub, GitLab's API relies on project IDs, while the GitLab website uses project paths. This distinction necessitated implementing functionality to support both behaviors.

I faced some challenges setting up the project, as I was unfamiliar with tox.

Currently, I've managed to write tests only for the URL generation, not for the Fetcher itself. I'm unsure which existing tests cover similar functionality.

If you could provide some guidance, I would be eager to fully complete the merge request. I'm relatively new to contributing to open source and would appreciate your help.

Checklist

• [x] Read the contribution guidelines
• [x] Run make locally before pushing commits
• [x] Add tests for the relevant parts:
  • [x] API
  • [ ] CLI
  • [ ] flake8 plugin (normal mode)
  • [ ] flake8 plugin (offline mode)
• [x] Write documentation when there's a new API or functionality

[andreoliwa]

Hey, thanks for this PR! 🙏🏻 I'm a bit busy at the moment but I will take a look as soon as possible.

[suhanoves]

Thanks for the review! I was on vacation, I’ll fix everything soon, according to your comments