bpo-42385: [Enum] add `_generate_next_value_` to StrEnum (GH-23735)

The default for auto() is to return an integer, which doesn't work for `StrEnum`.  The new `_generate_next_value_` for `StrEnum` returns the member name, lower cased.
diff --git a/Lib/test/test_enum.py b/Lib/test/test_enum.py
index 7ca54e9..f245eb6 100644
--- a/Lib/test/test_enum.py
+++ b/Lib/test/test_enum.py
@@ -2179,6 +2179,12 @@ class Private(Enum):
         self.assertEqual(Private._Private__corporal, 'Radar')
         self.assertEqual(Private._Private__major_, 'Hoolihan')
 
+    def test_strenum_auto(self):
+        class Strings(StrEnum):
+            ONE = auto()
+            TWO = auto()
+        self.assertEqual([Strings.ONE, Strings.TWO], ['one', 'two'])
+
 
 class TestOrder(unittest.TestCase):