[3.0.x] Fixed #30902 -- Added __str__() for model choice enums.

Allows expected behavior when cast to str, also matching behaviour of
created instances with those fetched from the DB.

Thanks to Simon Charette, Nick Pope, and Shai Berger for reviews.

Backport of dbcd7b064e from master

tests/model_enums/tests.py

@@ -143,6 +143,12 @@ class ChoicesTests(SimpleTestCase):
                 APPLE = 1, 'Apple'
                 PINEAPPLE = 1, 'Pineapple'
 
+    def test_str(self):
+        for test in [Gender, Suit, YearInSchool, Vehicle]:
+            for member in test:
+                with self.subTest(member=member):
+                    self.assertEqual(str(test[member.name]), str(member.value))
+
 
 class Separator(bytes, models.Choices):
     FS = b'\x1c', 'File Separator'