Update examples and tests for Python 3.8 compatibility

[tommilligan]

See #86 See #74

This PR fixes up a couple of small issues encountered when upgrading to Python 3.8:

• updates examples in README.md to be compatible with Python 3.8
• updates test code to match the formatting example provided in the readme
• adds Python 3.8 testing to Travis and tox
• marks the package as Python 3.8 compatible in setup.py

[the-wondersmith]

Would love to see this available in the mainline PyPi package

[tommilligan]

@madzak is there anything else I can do to help this into master?

[browniebroke]

Had the same issue, and this made the trick, thanks @tommilligan! 👍

[madzak]

Hi terribly sorry for the HUGE delay but i really appreciate you adding 3.8!