debian: define extend-diff-ignore for dpkg-source
This makes it possible to build the source package from a hg checkout
(and a sdist) by having dpkg-source ignore files not distributed in
sdist.
debian: use pybuild and dh_sphinxdoc
Use pybuild because it's kind of standard nowadays.
Use sphinxdoc debhelper to have a Depends: libjs-sphinxdoc and common
files (JS/CSS) symlinked.