Changes around six usage

Discussion around #1200 highlighted that we must be aware of differences
between the versions of six that Django ships and the one we use.

In general, the external version of six is the best choice for Oscar,
because it allows to depend on a specific version of six. The multiple
Django versions we support each come with different versions of six,
which makes it unattractive to depend on them.

When interfacing directly with Django, it should be decided on a
case-by-case basis if it's more appropriate to use Django's supplied
version of six or the external one.

In the case of the UppercaseCharField, Django clearly documents what
needs to be done to get it to work with both Python 2 and 3 in
https://docs.djangoproject.com/en/1.6/howto/custom-model-fields/#the-subfieldbase-metaclass
Those instructions are the same for all versions of Django we support.
Hence it's been decided that it's best to switch to Django's recommended
way of declaring the metaclass, and using Django's version of six for
it.

oscar/core/compat.py

@@ -1,10 +1,10 @@
+import six
+from six.moves import map
 from django.conf import settings
 from django.contrib.auth.models import User
 from django.core.exceptions import ImproperlyConfigured
-from django.utils import six
 from django.utils.html import conditional_escape
 from django.utils.safestring import mark_safe
-from six.moves import map
 
 
 def get_user_model():

oscar/models/fields.py

@@ -1,7 +1,9 @@
+import six
+
 from django.core.exceptions import ImproperlyConfigured
 from django.db.models.fields import CharField, DecimalField, Field
 from django.db.models import SubfieldBase
-from django.utils import six
+from django.utils import six as django_six
 from django.utils.translation import ugettext_lazy as _
 from django.core.validators import MaxLengthValidator
 
@@ -62,12 +64,13 @@ class PositiveDecimalField(DecimalField):
         return super(PositiveDecimalField, self).formfield(min_value=0)
 
 
-# necessary for to_python to be called
-@six.add_metaclass(SubfieldBase)
-class UppercaseCharField(CharField):
+class UppercaseCharField(django_six.with_metaclass(SubfieldBase, CharField)):
     """
     A simple subclass of ``django.db.models.fields.CharField`` that
     restricts all text to be uppercase.
+
+    Defined with the with_metaclass helper so that to_python is called
+    https://docs.djangoproject.com/en/1.6/howto/custom-model-fields/#the-subfieldbase-metaclass  # NOQA
     """
 
     def to_python(self, value):

setup.py

@@ -43,6 +43,7 @@ setup(name='django-oscar',
           # We use the ModelFormSetView from django-extra-views for the basket
           # page
           'django-extra-views>=0.2,<0.7',
+          # Search support
           'django-haystack>=2.1.0',
           # Treebeard is used for categories
           'django-treebeard==2.0b2',
@@ -58,7 +59,9 @@ setup(name='django-oscar',
           # For manipulating search URLs
           'purl>=0.7',
           # For phone number field
-          'phonenumbers==5.9.2'
+          'phonenumbers==5.9.2',
+          # Python 2 & 3 compatibility helper
+          'six>=1.5.2',
       ],
       # tarballs for unreleased packages
       dependency_links = [