Py3

[karlcz]

I'm making this PR to make code review easier. It is not ready to merge yet.

This ports the service to py3, still under mod_wsgi. No attempt is made to retain py2 compatibility.

Status:

• [x] Manual test install on Fedora 29 environment w/ Python 3.7 (filesystem backend)
• [x] Travis test w/ Python 3.5 (filesystem backend)
• [ ] Review documentation and sample configs for any remaining Python 2 references
• [ ] Port s3 backend to Python 3.5+ (or 3.7 only if significantly easier?)