To create a 3.5 backport, I use the following commands (something like this, adapt names ;-):
cd ~/python/3.5 git checkout -b fix_something35 git cherry-pick -x commit_sha1 # maybe fix conflicts or make further changes make ./python -m test -v test_modified_test # or better: ./python -m test -j0 -rW gh_pr.sh # my script to open a PR
I consider that it's easy enough to not have a "backport to 3.5" label.
I asked to remove this label since it was misused by contributors who are not aware that 3.5 not longer accept bug fixes.
Tell me if you need help to do the backport.
Note: there are many tools to open a pull requests on GitHub, but my script is working well for my workload so I never tried any other script ;-)
Night gathers, and now my watch begins. It shall not end until my death.