fix build with setuptools-scm

pybuild does the right thing if we properly depend on -scm in the
control file (!)

debian/control

@@ -13,6 +13,7 @@ Build-Depends: debhelper (>= 9)
              , python-sphinx
              , python-sphinx-rtd-theme
              , dh-python
+             , python-setuptools-scm
              , gnupg
 XS-Python-Version: >= 2.7
 Standards-Version: 3.9.8

debian/rules

@@ -8,7 +8,7 @@
 export DH_OPTIONS
 
 %:
-	dh $@  --with=python2,sphinxdoc
+	dh $@  --with=python2,sphinxdoc --buildsystem=pybuild
 
 override_dh_auto_install:
 	dh_auto_install