I am not yet specifically asking for review on this (although @dhermes has already been providing feedback "as I go"). This is a PR at this point because I need to be able to thrash at CircleCI.

This is an attempt to fix the CI situation on this repository. In particular, this will hit the following goals:

• Move to parallel builds on a per-API basis.
• Move system tests into the individual API packages.
• Eliminate Travis, move CircleCI to 2.0

In doing this, I made several changes:

• Move from tox to nox (Python-based configuration is really useful for the multi-project situation)
• Move from pylint to flake8 (pylint does not work on Python 3.6, has multiple difficult-to-silence false positives)

This is not done. In particular, several things need to be put back into place:

• Automatic docs build.
• Automatic PyPI releasing on appropriate tags.
• Only testing APIs that have changed files.