[EN-1454] Namespace fix

Author: iamishalkin

dxfeed/wrappers/subscription.py

@@ -59,7 +59,7 @@ def event_type(self):
 
     @property
     def symbols(self):
-        return dxf_get_symbols(self.__sub)
+        return dxp.dxf_get_symbols(self.__sub)
 
     def add_symbols(self, symbols: Union[str, Iterable[str]]):
         """