TST: xfail dateutil > 2.6.1 tests (#18240)

xref #18141

(cherry picked from commit 40fd6b4)

Author: jreback

pandas/tests/indexes/datetimes/test_tools.py

@@ -960,6 +960,7 @@ def test_guess_datetime_format_nopadding(self):
         for dt_string, dt_format in dt_string_to_format:
             assert tools._guess_datetime_format(dt_string) == dt_format
 
+    @pytest.mark.xfail(reason="GH18141 - dateutil > 2.6.1 broken")
     def test_guess_datetime_format_for_array(self):
         tm._skip_if_not_us_locale()
         expected_format = '%Y-%m-%d %H:%M:%S.%f'