According to https://github.com/travis-ci/dpl/blob/master/lib/dpl/provider/pypi.rb , pypi deployer is hard-wired to invoke python, twine and pip. So you need to override those commands. Creating symlinks in ~/bin (or whatever is early on PATH) in before_deploy: comes to mind.
Oh, and running the build in a virtualenv (as it also overrides python and commands for installed scripts) is an all-time favorite 
(I completely forgot about that!)