fixed pg8000 module access in xbrldb plugin.

The location of connect and error items in the pg8000 module has changed slightly. There is no DBAPI or errors files any more. All of these items are accessed directly through pg8000.

You can checkout https://github.com/mfenniak/pg8000/blob/master/pg8000/__init__.py for the current pg8000 code I used to update SqlDB.py.