Repport the add_metaclass decorator to handle metaclass overwriting

Author: TheBuky

pipeline/decorator.py

@@ -0,0 +1,35 @@
+# -*- coding: utf-8 -*-
+from __future__ import unicode_literals
+
+""" 
+    This code is a part of django.utils.six on https://github.com/django/django/blob/stable/2.2.x/django/utils/six.py removed form Django 3.0
+    To keep the backward compatibility between python 2 and 3 the decorator need to be used as well, during the time we find a proper way to 
+    handle MetaClass overwright working on both versions (or dropping python 2 support).
+"""
+
+def with_metaclass(meta, *bases):
+    """Create a base class with a metaclass."""
+    # This requires a bit of explanation: the basic idea is to make a dummy
+    # metaclass for one level of class instantiation that replaces itself with
+    # the actual metaclass.
+    class metaclass(meta):
+
+        def __new__(cls, name, this_bases, d):
+            return meta(name, bases, d)
+    return type.__new__(metaclass, 'temporary_class', (), {})
+
+
+def add_metaclass(metaclass):
+    """Class decorator for creating a class with a metaclass."""
+    def wrapper(cls):
+        orig_vars = cls.__dict__.copy()
+        slots = orig_vars.get('__slots__')
+        if slots is not None:
+            if isinstance(slots, str):
+                slots = [slots]
+            for slots_var in slots:
+                orig_vars.pop(slots_var)
+        orig_vars.pop('__dict__', None)
+        orig_vars.pop('__weakref__', None)
+        return metaclass(cls.__name__, cls.__bases__, orig_vars)
+    return wrapper

pipeline/forms.py

@@ -3,11 +3,16 @@
 from __future__ import unicode_literals
 
 from django.contrib.staticfiles.storage import staticfiles_storage
-from django.utils import six
 from django.utils.functional import cached_property
+try:
+    from django.utils.six import iteritems
+except ImportError:
+    def iteritems(dictionary):
+        return dictionary.items()
 
 from .collector import default_collector
 from .conf import settings
+from .decorator import add_metaclass
 from .packager import Packager
 
 
@@ -168,7 +173,7 @@ def _get_css_files(cls, extra_files):
                                   media_type='css',
                                   extra_files=extra_files.get(media_target,
                                                               [])))
-            for media_target, media_packages in six.iteritems(css_packages)
+            for media_target, media_packages in iteritems(css_packages)
         )
 
     def _get_js_files(cls, extra_files):
@@ -233,7 +238,7 @@ def _get_media_files(cls, packager, media_packages, media_type,
         return source_files
 
 
-@six.add_metaclass(PipelineFormMediaMetaClass)
+@add_metaclass(PipelineFormMediaMetaClass)
 class PipelineFormMedia(object):
     """Base class for form or widget Media classes that use Pipeline packages.