add missing python dependency

this is implicitly pulled-in through the test depends on rosunit,
but fails with ros-infrastructure/bloom#649
which disables test dependencies.

utilities/roslz4/package.xml

@@ -15,10 +15,13 @@
 
   <buildtool_depend>catkin</buildtool_depend>
 
+
   <build_depend>lz4</build_depend>
+  <build_depend>python3</build_depend>
   <build_depend>cpp_common</build_depend>
 
   <run_depend>lz4</run_depend>
+  <run_depend>python3</run_depend>
 
   <test_depend>rosunit</test_depend>
 </package>