also update docstring and add test.

consists mainly of adjusting fixtures to ensure connections are closed
explicitly.  psycopg2cffi also handles unicode bind parameter
names differently than psycopg2, and seems to possibly have a little less
control over floating point values at least in one test which is
marked as a "fail", though will see if it runs differently on linux
than osx..
- changelog for psycopg2cffi, fixes #3052

levels; :meth:`.Connection.get_isolation_level`,
:attr:`.Connection.default_isolation_level`.
- enhance documentation inter-linkage between new accessors,
existing isolation_level parameters, as well as in
the dialect-level methods which should be fully covered
by Engine/Connection level APIs now.

now be constructed such that the visit methods receive an indication
of the owning expression object, if any.  Any visit method that
accepts keyword arguments (e.g. ``**kw``) will in most cases
receive a keyword argument ``type_expression``, referring to the
expression object that the type is contained within.  For columns
in DDL, the dialect's compiler class may need to alter its
``get_column_specification()`` method to support this as well.
The ``UserDefinedType.get_col_spec()`` method will also receive
``type_expression`` if it provides ``**kw`` in its argument
signature.
fixes #3074

expression in an :class:`.Index` that did not correspond directly
to a table-bound column; typically when a :func:`.text` construct
was one of the expressions within the index; or could misinterpret the
list of expressions if one or more of them were such an expression.
fixes #3174

get PG stuff working

through all possible set values

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