gstlal: temporary hack for python elements

- elements must be installed using python3, but rest of this package
  requires python2 so we can't switch configure.ac over to it yet.  put a
  temporary hack into the gst/python/Makefile.am script to make it use the
  python3 interpreter during module install and byte-compile

gstlal/gst/python/Makefile.am

 # Put these Python scripts into the plugin directory under "python" so that
 # GStreamer sees them.
+# FIXME: elements must be byte-compiled with python 3.  remove when
+# configure.ac is picking up python3 for us
+PYTHON = $(shell env which python3)
 pkgpythondir = $(plugindir)/python
 pkgpyexecdir = $(pkgpythondir)