Make test_astype_str more specific

Author: evanpw

pandas/tests/test_series.py

@@ -5435,12 +5435,14 @@ def test_astype_str(self):
 
         # GH9757
         ts = Series([Timestamp('2010-01-04 00:00:00')])
-        ts = ts.astype(str)
-        self.assertEqual(ts.dtype, np.object_)
+        s = ts.astype(str)
+        expected = Series([str(ts.values[0])])
+        assert_series_equal(s, expected)
 
         td = Series([Timedelta(1, unit='d')])
-        td = td.astype(str)
-        self.assertEqual(td.dtype, np.object_)
+        s = td.astype(str)
+        expected = Series([str(td.values[0])])
+        assert_series_equal(s, expected)
 
     def test_astype_unicode(self):