HLTUtils: Fix for python 2 compatibility.

Make sure getoutput() is in the subprocess module.
We still have a python 2 build, so we want thigs to work with both 
py2 and py3.

HLT/HLTUtils/share/make_hlt_rep.py

 #!/usr/bin/env python
-# Copyright (C) 2002-2018 CERN for the benefit of the ATLAS collaboration
+# Copyright (C) 2002-2020 CERN for the benefit of the ATLAS collaboration
 
 import optparse
 import os
 
+# Make sure getoutput() is in the subprocess module.
+from future import standard_library
+standard_library.install_aliases()
+
 import pm.project
 import pm.common