Updates for Trac 1.4, 1.5 and Python 3
(Re-)Based on the existing '1.4_changes' branch, updated for newer Trac APIs and slightly modernized with the goal of having code that is compatible with both Python 2 and Python 3.
At the moment this is only rudimentarily tested (with Python 2.7.18/Trac 1.4.3 and Python 3.11/Trac 1.5.4dev), and without special plugins.
Maybe this could serve as a starting point for #1.
Comments welcome!