Add a job for Python 3.9 to .travis.yml

[Gallaecio]

It looks like Travis support specifying such a Python version as 3.9-dev.

While I’m not sure we should officially support Python 3.9 it until its release, running tests on it will allow us to catch any issue early.

[itsvinayak]

#4759 @Gallaecio, please review and some testcases are failing, can you please help

thank you,
@itsvinayak