Note that it requires the latest PyGreSQL 5.0
to pass all tests, a warning is printed otherwise.

Minor change to test.dialect.postgresql.test_types:
- JSON content should not be required to be unicode
  if the dialect doesn't return unicode (Python 2).
Minor change to dialects/postgresql.base:
- Return index names properly casted to unicode

The dialect may be initialized without the dbapi module being loaded.
Also, a connection may be made with no host argument set (localhost).

on unicode.   Enum would be better as a TypeDecorator
at this point but then that becomes awkward with native enum
types (Interval works that way, but we don't need the bind_processor for
native interval...)

fixes #3095, #3292
- reorganize enum constructor to again work with the MySQL
ENUM type
- add a new create_constraint flag to Enum to complement that of
Boolean
- reinstate the CHECK constraint tests for enum, these already
fail /skip against the MySQL backend
- simplify lookup rules in Enum, have them apply to all varieties
of Enum equally

removed; this has emitted a warning for many years and projects
should be calling upon ``sqlalchemy.dialects.postgresql``.
Engine URLs of the form ``postgres://`` will still continue to function,
however.

as subqueries in order to work around SQLite's lack of support for this
syntax, is lifted when SQLite version 3.7.16 or greater is detected.
fixes #3634
- The workaround for SQLite's unexpected delivery of column names as
``tablename.columnname`` for some kinds of queries is now disabled
when SQLite version 3.10.0 or greater is detected.
fixes #3633