fixed a bug in loadBefore

I guess there were no tests of GC.

That was made effective by recent changes in ZODB that relied more on
loadBefore for MVCC.

Oddly the change makes this loadBefore match the version it was copied
from, which github says was last changed 13 years ago. (gulp, on
multiple levels).

Test on pypy3 as well

It was already tested locally via tox.

See: http://blog.travis-ci.com/2014-12-17-faster-builds-with-container-based-infrastructure/

Downloaded from http://downloads.buildout.org/2/bootstrap.py

Python 3 compatibility