Update metaclass passing to work on Python 3

Fixes transformation labels in view titles.

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

HISTORY.rst

@@ -1,7 +1,8 @@
 3.3.4 (2019-12-XX)
 ==================
 - Update the gunicorn worker class to synchronous.
-
+- Update the way the BaseTransformationType metaclass is passed
+  to work on Python 3.
 
 3.3.3 (2019-12-05)
 ==================

mayan/apps/converter/transformations.py

@@ -18,7 +18,7 @@ class BaseTransformationType(type):
         return force_text(self.label)
 
 
-class BaseTransformation(object):
+class BaseTransformation(object, metaclass=BaseTransformationType):
     """
     Transformation can modify the appearance of the document's page preview.
     Some transformation available are: Rotate, zoom, resize and crop.
@@ -27,6 +27,7 @@ class BaseTransformation(object):
     name = 'base_transformation'
     _layer_transformations = {}
     _registry = {}
+    # PY 2
     __metaclass__ = BaseTransformationType
 
     @staticmethod

mayan/apps/converter/views.py

@@ -80,7 +80,7 @@ class TransformationCreateView(
                 '"%(transformation)s" for: %(object)s'
             ) % {
                 'layer': self.layer,
-                'transformation': self.get_transformation_class().label,
+                'transformation': self.get_transformation_class(),
                 'object': self.external_object,
             }
         }