Use six everywhere possible

[kashikoibumi]

Use six everywhere possible. This patch is one of preparations to migrate to Python3.

• use six.PY2 and six.PY3
• use six.itervalues(), six.iteritems()
• use six.int2byte(), six.byte2int, six.BytesIO
• use six.assertRaisesRegex, six.assertRegex, six.assertNotRegex
• use six.moves