TST: Add test for largest possible timedelta

Should be able to handle a 999,999,999 day timedelta because that is the
max allowed for the datetime library.

tests/test_time.py

@@ -119,7 +119,9 @@ def test_naturaldelta_nomonths(test_input, expected):
         (dt.timedelta(days=9), "9 days"),
         (dt.timedelta(days=365), "a year"),
         (dt.timedelta(days=365 * 1_141), "1,141 years"),
-        ("NaN", "NaN"),
+        ("NaN", "NaN"),  # Returns non-numbers unchanged.
+        # largest possible timedelta
+        (dt.timedelta(days=999_999_999), "2,739,726 years"),
     ],
 )
 def test_naturaldelta(test_input, expected):