populate sphinx "release" from git describe

[monkeypants]

the rendered documentation includes a "release" version code. Currently, it's fixed at a hardcoded value from conf.py.

It would be nice if it was a semantic version tag (see http://semver.org/). It would be nice if it was dynamically generated, i.e. automagically up to date. It would be nice if it was driven from git tags.

The solution has something to do with "git describe", tagging major and minor numbers manually, and allowing git to magically increment the patch number. The solution would also involve some code in the conf.py, and maybe adding some sort of git python package in requirements.txt