mkexp: fix handling of namelist section variables

mkexp

@@ -550,7 +550,8 @@ for subjob, subconfig in jobs_config.items():
         del subconfig
 
         # Extend all namelists
-        for namelist, groups in job_config['namelists'].items():
+        for namelist in job_config['namelists'].sections:
+            groups = job_config['namelists'][namelist]
             extended_groups = set()
             for group in groups.sections:
                 extend(group, groups, extended_groups)