Fix test scripts on some machines by specifying python3

Author: Ben Prather

tests/verify.sh

@@ -12,4 +12,4 @@ echo "Pixels different by >1% relative"
 h5diff -p 1.e-2 ../test-resources/${folder}_image.h5 image.h5 /pol
 
 # ...but we should only fail if this one returns nonzero
-python ../../scripts/compare.py ../test-resources/${folder}_image.h5 image.h5
+../../scripts/compare.py ../test-resources/${folder}_image.h5 image.h5