post_version.py fixes

The conversion to rST left this script in a broken state (that's mostly my bad); I've had these changes locally for weeks and have been using them when making releases, but I should really just upstream them.

post_version_test.py is also completely broken as it expects to modify html files and parse the result, not sure how we should fix it. My leading idea is to get docutils to generate the resulting html and test that; I'll see when I can get around to writing that.

/cc @dbaker