An implementation of RFC4013 SASLprep.
:Parameters:
- `data`: The string to SASLprep. Unicode strings
(python 2.x unicode, 3.x str) are supported. Byte strings
(python 2.x str, 3.x bytes) are ignored.
- `prohibit_unassigned_code_points`: True / False. RFC 3454
and RFCs for various SASL mechanisms distinguish between
`queries` (unassigned code points allowed) and
`stored strings` (unassigned code points prohibited). Defaults
to ``True`` (unassigned code points are prohibited).
:Returns:
The SASLprep'ed version of `data`.