WIP: Switch build-dependencies to python3

The tests are designed to work with either python2 or python3. We should be able to simply switch build-dependencies to python3 equivalents. However if something transitively pulls in python2 the AM_PATH_PYTHON autoconf macro will find python(2) binary, so in that case we need to pass PYTHON=python3 to dh_auto_configure. We try to avoid this by bumping the gtk-doc-tools build-dep to > 1.29 since that version has been switched from python2 to python3.