Add a has_key() method to the config classes for third party plugins that have not been ported to python 3

src/calibre/utils/config.py

@@ -362,6 +362,9 @@ def refresh(self, clear_current=True):
             self.clear()
         self.update(d)
 
+    def has_key(self, key):
+        return dict.__contains__(self, key)
+
     def __getitem__(self, key):
         try:
             return dict.__getitem__(self, key)