debian: also build with python3

We build with Python 3 along with Python 2. The binary package will
contain the extension for both python version. This is to ease
transition until the mercurial package in Debian uses Python 3. The
documentation is still built with Python 2 because it needs Mercurial
(which is currently Python 2).

--HG--
branch : stable
2 jobs for topic/stable/debian in 12 minutes and 10 seconds (queued for 3 minutes and 33 seconds)
Status Name Job ID Coverage
  Test
passed flake8 #46132

00:00:13

passed tests #46133

00:11:57