* in P3, the iterator stepping method has become a dunder method (next
  -> __next__), the protocol has not changed, add a __next__ alias to
  the iterator-next definition (nota: ir.config also has a next method
  but it's not part of an iterator, so don't alias it and don't
  convert calls to it)
* since ~2.6, a builtin (next()) has been added to step an iterator &
  allow for a default (in case of iterator end), convert all manual
  stepping to the builtin

Fixers:
    libfuturize.fixes.fix_next_call

#8530