Add transformation metaclass in a way compatible

Use Django's with_metaclass to Work with Python 2 and Python 3.

Signed-off-by: Roberto Rosario <roberto.rosario@mayan-edms.com>

HISTORY.rst

@@ -18,6 +18,8 @@
 - Use TemplateField for the web link template.
 - Use TemplateField for smart links.
 - Add the ID and the URL to the checkout serializer.
+- Add BaseTransformationType metaclass in a way compatible with
+  Python 2 and Python 3.
 
 3.3.4 (2019-12-09)
 ==================

mayan/apps/converter/transformations.py

@@ -6,6 +6,7 @@ import logging
 from PIL import Image, ImageColor, ImageDraw, ImageFilter
 
 from django.utils.encoding import force_bytes, force_text
+from django.utils.six import with_metaclass
 from django.utils.translation import string_concat, ugettext_lazy as _
 
 from .layers import layer_saved_transformations
@@ -18,7 +19,7 @@ class BaseTransformationType(type):
         return force_text(self.label)
 
 
-class BaseTransformation(object, metaclass=BaseTransformationType):
+class BaseTransformation(with_metaclass(meta=BaseTransformationType)):
     """
     Transformation can modify the appearance of the document's page preview.
     Some transformation available are: Rotate, zoom, resize and crop.
@@ -27,8 +28,6 @@ class BaseTransformation(object, metaclass=BaseTransformationType):
     name = 'base_transformation'
     _layer_transformations = {}
     _registry = {}
-    # PY 2
-    __metaclass__ = BaseTransformationType
 
     @staticmethod
     def combine(transformations):