change shebang in gstlal-inspiral stats tests to point at python3

gstlal-inspiral/tests/stats_horizonhistory_verify.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import doctest, sys
 from gstlal.stats import horizonhistory

gstlal-inspiral/tests/stats_trigger_rate_verify.py

-#!/usr/bin/env python
+#!/usr/bin/env python3
 
 import doctest
 import random