Setup python 3.8 builds
As the python 3.8 release date draws closer ( currently [scheduled](https://www.python.org/dev/peps/pep-0569/#release-schedule) for October this year ).

I was wondering if we want to set up python3.8 builds?

Given the 3.8 dev branch is available on [travis](https://docs.travis-ci.com/user/languages/python/) and shortly [azure](https://github.com/Microsoft/azure-pipelines-image-generation/issues/905).

Might be abit early for this - but it will mean we can catch any potential issues early + support python 3.8 upon release date.

Related issues: https://github.com/pandas-dev/pandas/issues/22714

I am happy to work on this - but would be good to get people's thoughts first.

Thanks!