Modernized enumeration helpers on Python 3.11+.

- use @enum.property

https://docs.python.org/3/library/enum.html#enum.property

- use @enum.nonmember

Using @property on an enum class does not yield the expected result.
do_not_call_in_templates attribute works because a @property instance
is truthy. We can make this a literal True value as expected by using
@enum.nonmember in Python 3.11+.

https://docs.python.org/3/library/enum.html#enum.nonmember

- used enum.IntEnum/StrEnum

Python 3.11+ has ReprEnum which uses int.__str__() and str.__str__()
for __str__() in the `IntEnum` and `StrEnum` subclasses. We can emulate
that for Python < 3.11.

https://docs.python.org/3/library/enum.html#enum.ReprEnum
https://docs.python.org/3/library/enum.html#enum.IntEnum
https://docs.python.org/3/library/enum.html#enum.StrEnum

tests/model_enums/tests.py

@@ -9,6 +9,7 @@ from django.test import SimpleTestCase
 from django.utils.deprecation import RemovedInDjango60Warning
 from django.utils.functional import Promise
 from django.utils.translation import gettext_lazy as _
+from django.utils.version import PY311
 
 
 class Suit(models.IntegerChoices):
@@ -187,6 +188,7 @@ class ChoicesTests(SimpleTestCase):
     def test_do_not_call_in_templates_member(self):
         # do_not_call_in_templates is not implicitly treated as a member.
         Special = models.IntegerChoices("Special", "do_not_call_in_templates")
+        self.assertIn("do_not_call_in_templates", Special.__members__)
         self.assertEqual(
             Special.do_not_call_in_templates.label,
             "Do Not Call In Templates",
@@ -197,6 +199,16 @@ class ChoicesTests(SimpleTestCase):
             "do_not_call_in_templates",
         )
 
+    def test_do_not_call_in_templates_nonmember(self):
+        self.assertNotIn("do_not_call_in_templates", Suit.__members__)
+        if PY311:
+            self.assertIs(Suit.do_not_call_in_templates, True)
+        else:
+            # Using @property on an enum does not behave as expected.
+            self.assertTrue(Suit.do_not_call_in_templates)
+            self.assertIsNot(Suit.do_not_call_in_templates, True)
+            self.assertIsInstance(Suit.do_not_call_in_templates, property)
+
 
 class Separator(bytes, models.Choices):
     FS = b"\x1c", "File Separator"