[1.11.x] Fixed #28269 -- Fixed Model.__init__() crash on models with a field that has an instance only descriptor.

Regression in d2a26c1a90.

Backport of ed244199c7 from master

tests/model_meta/models.py

@@ -9,6 +9,13 @@ class Relation(models.Model):
     pass
 
 
+class InstanceOnlyDescriptor(object):
+    def __get__(self, instance, cls=None):
+        if instance is None:
+            raise AttributeError('Instance only')
+        return 1
+
+
 class AbstractPerson(models.Model):
     # DATA fields
     data_abstract = models.CharField(max_length=10)
@@ -43,6 +50,8 @@ class AbstractPerson(models.Model):
     def test_property(self):
         return 1
 
+    test_instance_only_descriptor = InstanceOnlyDescriptor()
+
 
 class BasePerson(AbstractPerson):
     # DATA fields

tests/model_meta/tests.py

@@ -276,4 +276,6 @@ class ParentListTests(SimpleTestCase):
 
 class PropertyNamesTests(SimpleTestCase):
     def test_person(self):
+        # Instance only descriptors don't appear in _property_names.
+        self.assertEqual(AbstractPerson().test_instance_only_descriptor, 1)
         self.assertEqual(AbstractPerson._meta._property_names, frozenset(['pk', 'test_property']))