Add Python 3.9 on Travis

[liam-m]

No description provided.

[liam-m]

Getting

�[33;1mDownloading archive: https://storage.googleapis.com/travis-ci-language-archives/python/binaries/ubuntu/16.04/x86_64/python-3.9.tar.bz2�[0m
travis_time:start:07cb13b0
�[0K$ curl -sSf --retry 5 -o python-3.9.tar.bz2 ${archive_url}
curl: (22) The requested URL returned error: 404 Not Found

Other people are reporting the same thing at https://travis-ci.community/t/python-3-9-support/6703.