moved metaclass declaration into class descriptor.

pypatterns/behavioral/strategy.py

@@ -1,14 +1,13 @@
 from abc import ABCMeta, abstractmethod
 
 
-class Strategy(object):
+class Strategy(object, metaclass=ABCMeta):
     """
     An abstract Strategy class.
     All strategies should inherit this class.
 
     External Strategy Pattern documentation: U{https://en.wikipedia.org/wiki/Strategy_pattern}
     """
-    __metaclass__ = ABCMeta
 
     @abstractmethod
     def __call__(self, *args, **kwargs):