DROP TYPE instruction when a plain ``table.drop()`` is called,
assuming the object is not associated directly with a
:class:`.MetaData` object.   In order to accomodate the use case of
an enumerated type shared between multiple tables, the type should
be associated directly with the :class:`.MetaData` object; in this
case the type will only be created at the metadata level, or if
created directly.  The rules for create/drop of
Postgresql enumerated types have been highly reworked in general.
fixes #3319

For versions > 1.10.1 pg8000 returns de-serialized JSON objects rather
than a string. SQL parameters are still strings though.

pg8000 uses Versioneer, which means that development versions have
version strings that don't fit into the dotted triple number format.
Released versions will always fit the triple format though.

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

``pg_catalog.pg_table_is_visible(c.oid)``, rather than testing
for an exact schema match, when the schema name is None; this
so that the method will also illustrate that temporary tables
are present.  Note that this is a behavioral change, as Postgresql
allows a non-temporary table to silently overwrite an existing
temporary table of the same name, so this changes the behavior
of ``checkfirst`` in that unusual scenario.
fixes #3264