encoding: use range() instead of xrange()
Python 3 doesn't have xrange(). Instead, range() on Python 3
is a generator, like xrange() is on Python 2.
The benefits of xrange() over range() are when there are very
large ranges that are too expensive to pre-allocate. The code
here is only creating <128 values, so the benefits of xrange()
should be negligible.
With this patch, encoding.py imports safely on Python 3.