Switch to pybuild buildsystem

pybuild is a replacement for `python_distutils`, which requires
python2. pybuild is part of `dh-python`, so it will be available wherever our
build deps are installed.

See also:
 * https://wiki.debian.org/Python/LibraryStyleGuide#Overview
 * https://wiki.debian.org/Python/Pybuild

debian/rules

@@ -6,7 +6,7 @@ ifeq (,$(findstring nodoc, $(DEB_BUILD_OPTIONS)))
 endif
 
 %:
-	dh $@ --with python2 $(DH_ARGS)
+	dh $@ --buildsystem=pybuild --with python2 $(DH_ARGS)
 
 override_dh_auto_clean:
 	rm -rf doc/_build