Implement exporting of backups

[OpenBagTwo]

Summary

Resolves #10 via a new verb: gsb export which supports creating an archive of any revision in the Git history in either zip or tar (including compressed tar) format.

List of Changes

• Adds a standalone git archive implementation to the codebase that supports Zip files (see: https://github.com/libgit2/pygit2/issues/1239)
  • This module is licensed under GPLv2, as it borrows heavily from code existing in the GPLv2-licensed pygit2 library
• This allows export.export_backup to support for creating tar and zip archives, applying the appropriate format and compression based on the specified filename (which, in lieu of user-specification, is programmatically generated)
• Exposes a gsb export CLI that has support for specifying the archive format implicitly via filename or explicitly using a series of mutually-exclusive flags (which is a pain in the toucans to do with Click (see: https://github.com/pallets/click/issues/257)
  • I did look into using click-option-group but I must have been doing something wrong, because neither the documentation nor the actual mutual-exclusion checking were working as intended.

Tech Debt and Other Concerns

gsb export can be slow (for example, exporting a 3GB backup to a tar.xz). I don't believe tarfile supports multithreading, and I don't even know for certain that it doesn't require holding the entire tarball in memory during compression. Probably the best practice for power-users will be to export as uncompressed tar and then use xz standalone to perform the compression. This is something I may play with.

Export does not support ZIP archives that contain symlinks. That's a limitation of the zipfile library (see: https://bugs.python.org/issue37921). Given that ZIP is the default format for Windows users, I'm hoping that symlinks-in-backups is relatively rare / something that's easily mitigated by using a format flag.

Validation Performed

• Used the CLI to create backups of this project's Git repo at various tags and commits
  • generally this will error out for non-GSB repos, but if you provide the filename, it won't actually check for a manifest)
• Used the CLI to create an auto-named archive of an actual GSB-managed repo
• Confirmed that zip and tar archives show the same timestamps for their file contents (meaning I did https://github.com/OpenBagTwo/gsb/blob/f553fd82c02219d6404116e99f7aa61fcd0bdd4e/gsb/_make_zip_archive.py#L73-L81 correctly)
• [x] need to triple-check that Finder uncompresses .tar.gz files out-of-the-box with a single click. If that's not the case, I'll have to switch their default to either .tgz or .zip.

PR Type

• [ ] This PR introduces a breaking change (will require a bump in the minor version)
• [ ] The changes in this PR are high urgency and necessitate a hotfix or patch release (will require rebasing off of release)
• [ ] This is a release (staging) PR (maintainer use only)

Checklist:

• [x] I have read the contributor's guide
• [x] I have run mkdocs serve locally and ensured that all API docs and changes I have made to the static pages are rendering correctly, with all links working
• [x] All tech debt concerns have been resolved, documented as issues, or otherwise accepted

• ~~[ ] I agree to license my contribution to this project under the GNU Public License v3~~

  Technically GPLv2 is incompatible with GPLv3, which is part of the reason I've kept the module fully separate.

[OpenBagTwo]

Very pleased--and only slightly surprised--that all the various compression options are supported on Windows--I would not have been shocked if I needed to xfail some of those tests.