from six.moves import xrange for Python 3

Lines 1785 and 1818 contain calls to the Python 2-only builtin function __xrange()__ which was removed in Python 3 in favor of __range()__.  This PR adds the line [__from six.moves import xrange__](https://pythonhosted.org/six/#module-six.moves) for compatibility with both Python 2 and Python 3.