Test on Python 3.9

[Carreau]

Python 3.9 has been released, it is now time to start enabling it in CI once available.

[fandreuz]

I tried to work on the problem but I could not find the Travis configuration file. Any hint?

[Codesuha]

What's this project about?
How does one contribute. Pls tell

[jakirkham]

We would want to add it here

[joshmoore]

Note that this is in process in #859 since #858 points to a bug.