Fix warning in mustclose decorator

nilmdb/utils/mustclose.py

@@ -38,7 +38,8 @@ def must_close(errorfile = sys.stderr, wrap_verify = False):
 
         @wrap_class_method
         def close(orig, self, *args, **kwargs):
-            del self._must_close
+            if "_must_close" in self.__dict__:
+                del self._must_close
             return orig(self, *args, **kwargs)
 
         # Optionally wrap all other functions