'Convert ASCII-only unicode columns (dtype.kind=\'U\') to bytestring (dtype.kind=\'S\').
When exporting a unicode string array to a file in Python 3, it may be desirable
to encode unicode columns as bytestrings.  This routine takes advantage of numpy
automated conversion which works for strings that are pure ASCII.
The ``python3_only`` parameter is provided as a convenience so that code can
be written in a Python 2 / 3 compatible way::
>>> t.convert_unicode_to_bytestring(python3_only=True)
>>> t.write(\'my_data.fits\')
Parameters
python3_only : bool
Only do this operation for Python 3'

def convert_unicode_to_bytestring(self, python3_only=False):

    self._convert_string_dtype(u'U', u'S', python3_only)
