gstlal_ll_inspiral_trigger_counter: python 3 compatibility fixes

gstlal-inspiral/bin/gstlal_ll_inspiral_trigger_counter

-#!/usr/bin/env python
+#!/usr/bin/env python3
 #
 # Copyright (C) 2016  Kipp Cannon, Chad Hanna
 # Copyright (C) 2019  Patrick Godwin
@@ -23,7 +23,7 @@ import logging
 import sys, os
 import time
 import timeit
-import StringIO
+import io
 import threading 
 from gstlal import bottle
 import socket
@@ -119,7 +119,7 @@ class ZeroLagCounts(object):
 			# that you always get the most up to date.
 			zlpdf = self.zerolag_rankingstatpdf.copy()
 			self.__add_counts_to_zerolag_pdf(zlpdf)
-			output = StringIO.StringIO()
+			output = io.StringIO()
 			ligolw_utils.write_fileobj(self.__get_zerolag_rankingstatpdf_xmldoc(zlpdf), output)
 			outstr = output.getvalue()
 			output.close()