doing the isinstance() check - currently used only by psycopg2 + native enum + py2k.
- didn't realize psycopg2 had UNICODEARRAY extension all this time; replace _PGArray
with just using UNICODEARRAY instead.
- replace unnecessary/inconsistent __import__ in _isolation_lookup.

column argument.   If not a string, it checks that the object is
at least a :class:`.ColumnClause`, or an object that resolves to one,
and that the ``.table`` attribute, if present, refers to a
:class:`.TableClause` or subclass, and not something like an
:class:`.Alias`.  Otherwise, a :class:`.ArgumentError` is raised.
[ticket:2883]

have been modified, such that the COLLATE operator is now of lower
precedence than the comparison operators.  This has the effect that
a COLLATE applied to a comparison will not render parenthesis
around the comparison, which is not parsed by backends such as
MSSQL.  The change is backwards incompatible for those setups that
were working around the issue by applying :meth:`.Operators.collate`
to an individual element of the comparison expression,
rather than the comparison expression as a whole. [ticket:2879]

- get PG dialect to work around "no nonexistent binds" rule for now,
though we might want to reconsider this behavior

More fixes for cross references and reducing warnings (3rd wave)