Upgrade to pantsbuild.pants 0.0.21

This picks up support for t.c.python 0.6.0 and a broader
range of cheeseshops.

Adapt to package rework in pants apis (goal -> goal.goal, goal.phase)
in the t.c.pants plugins and prep them for a release with a minor
version bump and the start of a CHANGELOG.

https://rbcommons.com/s/twitter/r/763/