AFAIU 3.9.0 final is not yet available on Travis CI, but you can use 3.9-dev.
I’m not entirely certain how Travis adds support for Python versions. From past discussions I seem to remember that pyenv has to support 3.9 first, which appears to already have happened, so now what I think we need is a PR for https://github.com/travis-ci/cpython-builder/? If I understood the project structure, I’d try to create one…
UPDATE: ok, it seems to me no PR is necessary, just someone from Travis (last time it was @BanzaiMan) has to trigger a manual build with VERSION=3.9.0.