Ensure that we don't overlook errors in first PyObject_RichCompareBool call

Python 3.5 turns such cases into SystemErrors.

See: https://bugs.python.org/issue23571

Fixes #15.

Monte Carlo tests suck for reproducibility.

Human-readable __repr__ for pure-Python Bucket and Set objects.

Ensure that the changelog entry uses an imperative verb. You'd think I'd remember that by now. [skip ci]

Fix setting _p_changed when removing from small Python BTrees/TreeSets.

Pure Python Set objects were also not marking themselves changed on addition. This impacted things like zope.index.topic.filter

Make all the tests in the w_zodb Tox environment run.

Fix setting _p_changed for small pure-Python BTrees.

Fixes #9 (I hope).

[ci skip]

Lost 56bd1d04 in a merge?

[ci skip]