This is for people using dtuple.py; a dtuple.DatabaseTuple instance
keeps a reference to cursor.description, which is not picklable because
psycopg2 doesn't export the Column namedtuple it uses.
This commit exports the Column namedtuple, and includes a test to verify
the pickle/unpickle works after exporting Column.
TypeError is the standard Python error raised in this case:
$ python -c "(lambda a: None)(b=10)"
TypeError: <lambda>() got an unexpected keyword argument 'b'
We only used to raise InterfaceError when connect was used without
any parameter at all, so it's hard to think a program depending on
that design. Furthermore the function has always raised (and still
does) OperationalError too, if the bad argument is detected by the
libpq, and that cannot be changed because we can't tell the
difference from a normal connection error.
Failing to do so, the real cause of the _psycopg import failed may get
hidden and people may get a misleading error such as "cannot import name
tz" instead.
Added an adapter for None: it is usually not invoked as adaptation to
NULL is a fast path in mogrify, but can be invoked by composite types.
Notice that composite types still have the option to fast-path None
(e.g. list adapter does).
