corrected a syntax error: "raise AttributeError(key)" instead of "raise

AttributeError, key"

cherry-picked from: scholrly#5

Author: Matt Sinclair

dblp/__init__.py

@@ -18,7 +18,7 @@ def __getattr__(self, key):
             if self.data is None:
                 self.load_data()
             return self.data[key]
-        raise AttributeError, key
+        raise AttributeError(key)
 
     def load_data(self):
         pass