python3: fix incorrect use of merging two dictionaries

In python2, this was inefficient, because it allocated *three* lists of
tuples, before finally generating a dict based on them. In python3, it
fails because you cannot combine the dict_items() type.

Moreover, retval was always a function-local dictionary used purely for
returning the value, so dict1.update(dict2) will always yield the
correct result, and we don't even need to create a copy to avoid
mutating the original dictionary.

calibre-plugin/dialogs.py

@@ -491,7 +491,8 @@ class AddNewDialog(SizePersistedDialog):
                 retval['updatemeta']=True
                 retval['collision']=ADDNEW
 
-        return dict(retval.items() + self.extraoptions.items() )
+        retval.update(self.extraoptions)
+        return retval
 
     def get_urlstext(self):
         return unicode(self.url.toPlainText())