Add support for Trac 1.6 and Python3

[bmispelon]

This PR adds support for Python3 (>= 3.7) and Trac 1.6 (while retaining support for Python 2.7 and Trac >= 1.2).

I've tried to split it into several commits for easier review and/or cherry picking 🍒 I started with 3 commits (marked with [runtests]) that refactor the test suite to make the compatibility easier (the big one being replacing urllib2 with requests). Then I tried to make one [py3-compat] commit per change, except for the final one which I found too hard to split.

Let me know if there's anything that doesn't make sense, or that you'd like changed.

Once this is merged I should be able to contribute a fix for #136 and after that I would like to assist you in getting a release on PyPI if that's possible 🤞🏻

Thanks! ✨