Python 3 was released more than 10 years ago, and support for Python 2
is going away, with many Linux distributions starting to phase it out.
This patch migrates git-arr to Python 3.
The generated output is almost exactly the same, there are some minor
differences such as HTML characters being quoted more aggresively, and
handling of paths with non-utf8 values.
As an experiment, make the sections of the summary to be toggable. This
can help readability, although it's unclear if it's worth the additional
complexity and could be removed later.
Including the tree as part of the summary gives a bit more information
and provides an easy path into the tree.
It does clutter things a bit, so this is an experiment and may be
removed later.
