debian: hack to prevent trying to build python3 packages when unsupported

We have to trick py3versions into returning an empty string by requiring
nonexistent Python versions.
Solution proposed by Steffen Grunewald.
(cherry picked from commit bc75722d28ecebb92ac27bdc010add2068c26cd6)
Original: 1d6b50216566f985852d3f7e52940c9d85409002

lalapps/debian/control.in

@@ -27,7 +27,7 @@ Build-Depends: debhelper (>= 9), help2man, pkg-config, dh-python, python-all-dev
   lalinference-python (>= @MIN_LALINFERENCE_VERSION@~),
   bc
 X-Python-Version: >= 2.7
-X-Python3-Version: >= 3.4
+X-Python3-Version: <= 3.0
 Standards-Version: 3.9.8
 
 Package: lalapps

laldetchar/debian/control.in

@@ -18,7 +18,7 @@ Build-Depends: debhelper (>= 9), help2man, pkg-config, dh-python, python-all-dev
   lalburst-python (>= @MIN_LALBURST_VERSION@~),
   lalburst-octave (>= @MIN_LALBURST_VERSION@~)
 X-Python-Version: >= 2.7
-X-Python3-Version: >= 3.4
+X-Python3-Version: <= 3.0
 Standards-Version: 3.9.8
 
 Package: laldetchar

lalinference/debian/control.in

@@ -35,7 +35,7 @@ Build-Depends: debhelper (>= 9), help2man, pkg-config, dh-python, python-all-dev
   lalpulsar-python (>= @MIN_LALPULSAR_VERSION@~),
   lalpulsar-octave (>= @MIN_LALPULSAR_VERSION@~)
 X-Python-Version: >= 2.7
-X-Python3-Version: >= 3.4
+X-Python3-Version: <= 3.0
 Standards-Version: 3.9.8
 
 Package: lalinference