psycopg2cffi dialect, in particular that the current 2.7.0 version
does not have native support for the JSONB type.  The version detection
for psycopg2 features has been tuned into a specific sub-version
for psycopg2cffi.  Additionally, test coverage has been enabled
for the full series of psycopg2 features under psycopg2cffi.
fixes #3439

:func:`.engine_from_config` were not being parsed correctly;
these included ``pool_threadlocal`` and the psycopg2 argument
``use_native_unicode``. fixes #3435
- add legacy_schema_aliasing config parsing for mssql
- move use_native_unicode config arg to the psycopg2 dialect

The pg8000 dialect now supports the setting of the PostgreSQL parameter
client_encoding from create_engine().

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

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

As with postgresql+psycopg2,
execution_options(isolation_level='AUTOCOMMIT') now works for the
postgresql+pg8000 dialect.

Also enabled the autocommit test in test_dialect.py for pg8000.