I just saw your question I was wondering about this gitPython, looks like really nice tool, and I was looking for this specific question in the GitPython documentation with no lucky but if you search on github you will see a lot of tests there and there's a test for that.
You will see something like this when you search for "remove new branch":
# remove new branchHead.delete(new_remote_branch.repo, new_remote_branch)