Fixed #20430 - Enable iterable of iterables for model choices

Allows for any iterable, not just lists or tuples, to be used as
the inner item for a list of choices in a model.

tests/model_validation/models.py

@@ -0,0 +1,27 @@
+from django.db import models
+
+
+class ThingItem(object):
+
+    def __init__(self, value, display):
+        self.value = value
+        self.display = display
+
+    def __iter__(self):
+        return (x for x in [self.value, self.display])
+
+    def __len__(self):
+        return 2
+
+
+class Things(object):
+
+    def __iter__(self):
+        return (x for x in [ThingItem(1, 2), ThingItem(3, 4)])
+
+
+class ThingWithIterableChoices(models.Model):
+
+    # Testing choices= Iterable of Iterables
+    #   See: https://code.djangoproject.com/ticket/20430
+    thing = models.CharField(max_length=100, blank=True, choices=Things())