improve handling of choices
   _expand_choices - previous code in 2 places, now 1
      called in 3 places
   HelpFormatter - change mutexgroup filter to ignore 'range(20)'
Relate py development tries to deal with
 - long choice lists
 - choices that have __contains__ but not __iter__
   no direct JS equivalent.  This code tries to expand
   'choices' into an array to which it can apply 'in'