- The :class:`.mysql.SET` type has been overhauled to no longer

- The :class:`.mysql.SET` type has been overhauled to no longer
assume that the empty string, or a set with a single empty string
value, is in fact a set with a single empty string; instead, this
is by default treated as the empty set.  In order to handle persistence
of a :class:`.mysql.SET` that actually wants to include the blank
value ``''`` as a legitimate value, a new bitwise operational mode
is added which is enabled by the
:paramref:`.mysql.SET.retrieve_as_bitwise` flag, which will persist
and retrieve values unambiguously using their bitflag positioning.
Storage and retrieval of unicode values for driver configurations
that aren't converting unicode natively is also repaired.
fixes #3283