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

array.__init__() expects a list as its sole parameter but inside _bind_param(), instead of sending a list it's sending each item in the list as a separate argument which is incorrect.

where engine-level isolation level parameter would raise an error
on connect. fixes #3134

sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

The postgresql base dialect has problems with two-phase commit because
there isn't a standard way of handling autocommit in DBAPI. This commit
modifies the pg8000 dialect to use the DBAPI tpc extension, which is
supported by pg8000 as of version 1.9.11.

subclasses where direct association of the type with a
:class:`.MetaData` would lead to a hang when events
(like create events) were emitted on the :class:`.MetaData`.
fixes #3124

to get all flake8 passing