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).
debian: update debhelper compat
Level before 9 are deprecated says debhelper 12. Upgrade to 10, which is
available in Debian oldstable and old Ubuntu releases to be conservative
(and since we probably don't need more).