Ignore bulidout artifacts.
Update to latest buildout bootstrap.

Downloaded from https://bootstrap.pypa.io/bootstrap-buildout.py.
Require 100% coverage.
persistent.list:  100% branch coverage.
persistent.persistence:  100% branch coverage.
persistent.picklecache:  100% branch coverage.
Add branch coverage.

Exclude interfaces, whose method docstrings confuse the coverage checker.
Add support for PyPy3.