Six compatibility

Created by: stefanoborini

Introduced six compatibility for metaclass usage.

force_bdss/data_sources/base_data_source.py

 import abc
+import six
 
 
-class BaseDataSource(metaclass=abc.ABCMeta):
+class BaseDataSource(six.with_metaclass(abc.ABCMeta)):
     def __init__(self, bundle, application, model):
         self.bundle = bundle
         self.application = application