The branch name is used in the version switcher, so the master branch should have a version of "master".
Maybe the release could be the revision hash? Not sure how to extract it from the repo.
# built documents.
#
# The short X.Y version.
-version = '8.0'
+version = 'master'
# The full version, including alpha/beta/rc tags.
-release = '8.0b1'
+release = 'master'
# There are two options for replacing |today|: either, you set today to some
# non-false value, then it is used: