Add Python 3.9.0 to the testing

[takluyver]

Thanks! Do you know when we can just put 3.9 without the dev suffix?

[cclauss]

• https://storage.googleapis.com/travis-ci-language-archives/python/binaries/ubuntu/16.04/x86_64/python-3.9.tar.bz2
• https://docs.travis-ci.com/user/languages/python/#python-versions
• https://travis-ci.community/t/python-3-9-0-build/10091

[takluyver]

I was vaguely hoping that if I delayed a few days, plain 3.9 would be available. But it seems not, so let's merge anyway - thanks!