Reintroduce txtorcon's deb

Heard back from Iain. He indicated that python2 packages are being removed for
Debian's next release, but txtorcon's python3 package is still around.

package_versions.py

@@ -65,6 +65,7 @@ PACKAGES = [
     Package('netbsd', 'http://pkgsrc.se/net/py-stem', NETBSD_VERSION),
   ]),
   ('txtorcon', [
+    Package('debian', 'https://packages.debian.org/sid/python3-txtorcon', DEBIAN_VERSION),
     Package('gentoo', 'https://packages.gentoo.org/packages/dev-python/txtorcon', None),
     Package('archlinux', 'https://aur.archlinux.org/packages/python-txtorcon/', AUR_VERSION),
     Package('slackware', 'https://slackbuilds.org/repository/14.2/python/txtorcon/', 'txtorcon-([0-9\.]+).tar.gz'),