Fix regressions, related to xrange -> range conversion in PR #2318

Quote myself [from the cited above PR](https://github.com/sympy/sympy/pull/2318#issuecomment-21708875):
> This pull changes all xrange calls to range, instead of creating wrapper (as mpmath do). But there is a big difference for py2!
>
> I thought, that most of ours xrange's was intentional, isn't?

See also thread https://groups.google.com/forum/?fromgroups=#!topic/sympy/ZRAS1bizsAE

This pull reverts xrange's, except for cases like range(3) or range(0, 7).