rst: Make has_cjk() Python 3 compatible.

In Python 3, iterators no longer have a next() method: it has been renamed to
__next__(). This could be worked around by using the builtin function next(),
but a simpler fix is to just use the regex' object search() method instead of
finditer().
rst: Replace xrange() by Python 3 compatible range().
Merge pull request #408 from Oberon00/rst-fix-python3

rst: Improve compatibility of snippets with Python 3