Test on Python 3.8

[effigies]

Summary

Python 3.8 has been out for a month or so, and upstream libraries are mostly providing wheels. We should start 3.8 tests when it's feasible.

Necessary steps:

• Update .travis.yml to use Python 3.8
• Note any failures, identify whether they exist on our end or in a dependency
• Find blocking upstream issues and link to them so we can track
• When tests pass and urgent FutureWarning/DeprecationWarnings are dealt with, update the classifiers in nipype/info.py to indicate we support Python 3.8.

I marked this but it's less for a new developer than someone looking to get more involved in project maintenance.