[3.0.x] Refs #27914 -- Fixed serialization of nested enum.Enum classes in migrations.

Backport of 6452112640 from master

tests/migrations/test_writer.py

@@ -184,6 +184,9 @@ class WriterTests(SimpleTestCase):
     """
     Tests the migration writer (makes migration files from Migration instances)
     """
+    class NestedEnum(enum.IntEnum):
+        A = 1
+        B = 2
 
     def safe_exec(self, string, value=None):
         d = {}
@@ -289,6 +292,14 @@ class WriterTests(SimpleTestCase):
             IntEnum.B,
             ("migrations.test_writer.IntEnum['B']", {'import migrations.test_writer'})
         )
+        self.assertSerializedResultEqual(
+            self.NestedEnum.A,
+            (
+                "migrations.test_writer.WriterTests.NestedEnum['A']",
+                {'import migrations.test_writer'},
+            ),
+        )
+        self.assertSerializedEqual(self.NestedEnum.A)
 
         field = models.CharField(default=TextEnum.B, choices=[(m.value, m) for m in TextEnum])
         string = MigrationWriter.serialize(field)[0]