Make the BaseCache class an abstract class.

This is just a matter of cleanup, I can't think of any good reason for this
_not_ to be marked abstract.

Author: Flameeyes

cachecontrol/cache.py

@@ -6,18 +6,26 @@
 The cache object API for implementing caches. The default is a thread
 safe in-memory dictionary.
 """
+
+from abc import ABCMeta, abstractmethod
 from threading import Lock
 
+from six import add_metaclass
+
 
+@add_metaclass(ABCMeta)
 class BaseCache(object):
+    @abstractmethod
     def get(self, key):
-        raise NotImplementedError()
+        pass
 
+    @abstractmethod
     def set(self, key, value):
-        raise NotImplementedError()
+        pass
 
+    @abstractmethod
     def delete(self, key):
-        raise NotImplementedError()
+        pass
 
     def close(self):
         pass