[unittest] Handle case where pint is not installed

test/python/test_units.py

@@ -4,7 +4,7 @@
 import sys
 
 import pytest
-import cantera.with_units as ctu
+ctu = pytest.importorskip("cantera.with_units")
 import cantera as ct
 try:
     from pint.testing import assert_allclose