Log Message:
www/trac: Update to 1.5.4

This update has been build-tested only; no one responded on
pkgsrc-users about testing or doing the update.

Packaging changes:
  Switch to python3.

Upstream changes:

  Support python 3.5+.  Drop support for python 2.7.
  Drop Genshi; plugins must be migrated to Jinja2.
  Allow retargeting tickets from the milestone admin page.