Fix naturaldelta tests with missing timezone info

tests/test_time.py

@@ -349,13 +349,14 @@ def test_naturaldelta_when_explicit(test_input, when, expected):
     "value, when",
     [
         (NOW_UTC, None),
-        (NOW_UTC, NOW_UTC_PLUS_01_00),
-        (NOW_UTC_PLUS_01_00, NOW_UTC),
+        (NOW_UTC, NOW),
+        (NOW_UTC_PLUS_01_00, None),
+        (NOW_UTC_PLUS_01_00, NOW),
     ],
 )
-def test_naturaldelta_when_mixed_timezones(value, when):
-    """Mixed timezones on `value` and `when` should lead to a `TypeError`,
-    since subtraction `when - value` is not defined by the `datetime` module.
+def test_naturaldelta_when_missing_tzinfo(value, when):
+    """Subtraction `when - value` is not defined by the `datetime` module when
+    either operand has not timezone-info (`tz=None`) and raises a TypeError.
     """
 
     # Act / Assert