Attempt to pull new tags if fetching a tag fails

[eliminmax]

Checklist

• [x] Closing issues: #36
• [x] Mark this if you consider it ready to merge
• [ ] I've added tests (optional)
• [ ] I wrote some documentation

Description

I edited the git_fetch function in tools.py, so that if repo.git.checkoit(version) fails with a git.GitCommandError, it tries to fetch tags from origin and tries one more time, instead of immediately failing.

I am not super familiar with the GitPython module, but I did not see anything in the docs that would point to cleaner way to do things than pass the --tags flag.