Test against Python 3.9 on Travis CI

[McSinyx]

Continue stabbing at GH-8967 (yes I'm craving for violence tonight!). Marked as draft until Travis is ready.

[McSinyx]

I think this can be merged now. I still can't figure out what @pypa-bot wants so probably a maintainer needs to flag this as trivial.

[pfmoore]

At least one of the changes to test on 3.9 probably deserves a proper news entry. But it doesn't have to be this one. You can add a "trivial" news file, or I can add the label, but let's work out which PR will get the news entry before we accidentally mark them all as trivial 🙂

[pradyunsg]

There is a "trivial" file in the news/ directory of this PR, and looking at the bot's logs, I'm still not sure why things aren't working.

[pfmoore]

Sorry, so there is. I can't see the bot logs, so I don't know. Maybe some sort of timing over the recent change to require a .rst suffix on trivial news files?

I've added the label and will merge. But we should still make sure that #8967 gets a news entry somewhere.

[McSinyx]

Thanks, and is the following enough?

pip/news/8971.feature.rst

Line 1 in da7569a

Support Python 3.9.

When I wrote it I noticed that for introduction for support of past CPython version are quite brief also, e.g. the one for py38.

[pfmoore]

Yes, something brief is reasonable. I would go with something like "Add 3.9 to the versions tested in CI", to be explicit that it's about CI.

[McSinyx]

I assume that we should use the process news fragment, is that correct? ^{Yes I've been waiting for a chance to use it since forever!}

[pfmoore]

I knew there was a process one, but when I looked I couldn't see it mentioned. Yes, that's the one to use.

[McSinyx]

I've added the news fragment to GH-9013.