[Test] Skip pint-based tests for pint<0.17.0

The test helpers we use for the pint-based tests require pint 0.17
or newer. This requirement doesn't extend to the cantera.with_units
module, though, which works with with versions at least as far back
as 0.10.1.

test/python/test_units.py

@@ -4,12 +4,14 @@
 import sys
 
 import pytest
-ctu = pytest.importorskip("cantera.with_units")
+pytest.importorskip("pint", "0.17.0")
+import cantera.with_units as ctu
 import cantera as ct
 try:
     from pint.testing import assert_allclose
 except ModuleNotFoundError:
     # pint.testing was introduced in pint 0.20
+    # assert_quantity_almost_equal was introduced in pint 0.17.
     from pint.testsuite.helpers import assert_quantity_almost_equal as assert_allclose