Provide opportunity to get enums list via an inspector instance public
interface.

otherwise render a SQL NULL column value, rather than a JSON-encoded
``'null'``.  To support this case, changes are as follows:

* The value :func:`.null` can now be specified, which will always
  result in a NULL value resulting in the statement.

* A new parameter :paramref:`.JSON.none_as_null` is added, which
  when True indicates that the Python ``None`` value should be
  peristed as SQL NULL, rather than JSON-encoded ``'null'``.

Retrival of NULL as None is also repaired for DBAPIs other than
psycopg2, namely pg8000.

fixes #3159

non-standard DBAPI exceptions, such as the psycopg2
TransactionRollbackError.  These exceptions will now be raised
using the closest available subclass in ``sqlalchemy.exc``, in the
case of TransactionRollbackError, ``sqlalchemy.exc.OperationalError``.
fixes #3075

simplify tox again now that we can exclude tests more easily

to a plain Python list would fail to use the correct array constructor.
Pull request courtesy Andrew.  fixes #3141

- add __backend__ to most tests so that pg8000 can start coming in

'username' was assumed to be in the database URL, even though
this might not be the case.  Fixes #3128

it and move on