BUG: Fix __classcell__ propagation.

Python 3.6 added simpler customization of class creation but this
requires to propagate correctly the __classcell__ attribute in custom
__new__ methods.

See https://docs.python.org/3.6/whatsnew/3.6.html#pep-487-simpler-
customization-of-class-creation

scrapy/item.py

@@ -25,6 +25,7 @@ class Field(dict):
 class ItemMeta(ABCMeta):
 
     def __new__(mcs, class_name, bases, attrs):
+        classcell = attrs.pop('__classcell__', None)
         new_bases = tuple(base._class for base in bases if hasattr(base, '_class'))
         _class = super(ItemMeta, mcs).__new__(mcs, 'x_' + class_name, new_bases, attrs)
 
@@ -39,6 +40,8 @@ class ItemMeta(ABCMeta):
 
         new_attrs['fields'] = fields
         new_attrs['_class'] = _class
+        if classcell is not None:
+            new_attrs['__classcell__'] = classcell
         return super(ItemMeta, mcs).__new__(mcs, class_name, bases, new_attrs)
 
 

tests/test_item.py

@@ -1,8 +1,14 @@
+import sys
 import unittest
 
-from scrapy.item import Item, Field
 import six
 
+from scrapy.item import ABCMeta, Item, ItemMeta, Field
+from tests import mock
+
+
+PY36_PLUS = (sys.version_info.major >= 3) and (sys.version_info.minor >= 6)
+
 
 class ItemTest(unittest.TestCase):
 
@@ -244,5 +250,49 @@ class ItemTest(unittest.TestCase):
         self.assertNotEqual(item['name'], copied_item['name'])
 
 
+class ItemMetaTest(unittest.TestCase):
+
+    def test_new_method_propagates_classcell(self):
+        new_mock = mock.Mock(side_effect=ABCMeta.__new__)
+        base = ItemMeta.__bases__[0]
+
+        with mock.patch.object(base, '__new__', new_mock):
+
+            class MyItem(Item):
+                if not PY36_PLUS:
+                    # This attribute is an internal attribute in Python 3.6+
+                    # and must be propagated properly. See
+                    # https://docs.python.org/3.6/reference/datamodel.html#creating-the-class-object
+                    # In <3.6, we add a dummy attribute just to ensure the
+                    # __new__ method propagates it correctly.
+                    __classcell__ = object()
+
+                def f(self):
+                    # For rationale of this see:
+                    # https://github.com/python/cpython/blob/ee1a81b77444c6715cbe610e951c655b6adab88b/Lib/test/test_super.py#L222
+                    return __class__
+
+            MyItem()
+
+        (first_call, second_call) = new_mock.call_args_list[-2:]
+
+        mcs, class_name, bases, attrs = first_call[0]
+        assert '__classcell__' not in attrs
+        mcs, class_name, bases, attrs = second_call[0]
+        assert '__classcell__' in attrs
+
+
+class ItemMetaClassCellRegression(unittest.TestCase):
+
+    def test_item_meta_classcell_regression(self):
+        class MyItem(six.with_metaclass(ItemMeta, Item)):
+            def __init__(self, *args, **kwargs):
+                # This call to super() trigger the __classcell__ propagation
+                # requirement. When not done properly raises an error:
+                # TypeError: __class__ set to <class '__main__.MyItem'>
+                # defining 'MyItem' as <class '__main__.MyItem'>
+                super(MyItem, self).__init__(*args, **kwargs)
+
+
 if __name__ == "__main__":
     unittest.main()