This requirement is needed to ensure a total ordering, e.g. in Py3k where
strings and ints are not comparable.

Suppress warning under 'tox -e docs'

Maybe better to delete it?  It was there to support the no-longer-future
ZODB4 commit model.

Note that we now raise ValueError from doom() if not in a doomable state,
rather than asserting.

Note that we now raise ValueError if passed a non-positive count, rather
than asserting.