fix syntax error from previous

Author: keflavich

astroquery/vizier/core.py

@@ -538,7 +538,7 @@ def _args_to_payload(self, *args, **kwargs):
             columns = self.columns + columns
             # filter columns to _unique_ columns, preserving order in python >3.6
             # note that "set" does not preserve order, but dict.keys does
-            columns = list(dict(columns).keys())
+            columns = list(dict.fromkeys(columns, ).keys())
 
         # special keywords need to be treated separately
         # keyword names that can mean 'all'