Attempt at Py3 compatibility

[matthewblain]

This includes several changes:

• Remove unused PyDNS dependency.
• Change dependency on dnspython to one of dnspython or dnspython3.
• Change urlparse to a py2/py3 works-in-both version import.
• Change to zope.interface.implementer.

At least one dependency (pyramid_simpleform) is 5 years out of date on PyPi, so it doesn't support Py3. Though it appears fine in the current master on github.

pyramid_basemodel generate_random_digest also needs a small change for py3 support. so does pyramid_simpleauth\model.py . (Both should use bytes, not unicode.)

passlib has supported python3 for some time now, that does not appear to cause issues..

I'm going to stop here, but for the next person who attempts this they'll have less work to do.