Begin testing early versions of Python 3.8 on one CI system

[tleonhardt]

Now that Python 3.8.0 beta1 has been released and 3.8.0 has hit feature freeze we should begin unit testing on a development version of 3.8 on one of our CI systems.

[Travis CI](travis ci python 3.8) should have support for 3.8-dev.

[tleonhardt]

Once tests for 3.8 are passing, we should update our setup.py and documentation to support 3.8.