Bug #1520864 (again): unpacking singleton tuples in list comprehensions and

Bug #1520864 (again): unpacking singleton tuples in list comprehensions and
generator expressions (x for x, in ... ) works again.

Sigh, I only fixed for loops the first time, not list comps and genexprs too.
I couldn't find any more unpacking cases where there is a similar bug lurking.

This code should be refactored to eliminate the duplication.  I'm sure
the listcomp/genexpr code can be refactored.  I'm not sure if the for loop
can re-use any of the same code though.

Will backport to 2.5 (the only place it matters).