Add git tag for 0.91 (PyPI) release

[koobs]

PyPI is currently at 0.91 but there isn't a corresponding git tag for this version.

Please add a 0.91 git tag for the revision from which the last PyPI sdist was produced would be appreciated

I'm creating a FreeBSD port, and will be using GitHub to fetch sources as the sdist is missing some desirable files (will create separate issues, PR's for these). When using GitHub for sources, its preferable to have exact git tags from which to fetch the tarballs, rather than using commit revision hashes