revert no-longer-necessary

Author: jbrockmendel

Diff for: pandas/tests/indexes/datetimes/methods/test_astype.py

@@ -171,7 +171,7 @@ def test_astype_datetime64(self):
         tm.assert_index_equal(result, idx)
         assert result is not idx
 
-        result = idx.astype("datetime64[s]", copy=False)
+        result = idx.astype("datetime64[ns]", copy=False)
         tm.assert_index_equal(result, idx)
         assert result is idx