Get rid of dependencies on z3c.testing and zope.app.testing.

The first was achieved by duplicating some test setup code,
and the second by using testing helpers from zope.site and
zope.traversing instead of placefulSetUp.
Added a tox config and a manifest.
New bootstrap.
Trivial part of Python 3 porting.
Stop relying on dictionary key order.
Fix non-deterministic failure on Py3.

On Python 3 the order of adapters returned by getAdapters is unpredictable, so
columns with the same weight jump around.
Py3 support.
Silence a warning.
Update changelog, add supported Python versions to setup.py.