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

that have problems with right-nested joins and UNION column keys;
references #3633 references #3634.   backport from 1.1 to 0.9
announcing 1.1 as where these behaviors will be retired based
on version-specific checks
- fix test_resultset so that it passes when SQLite 3.10.0 is
present, references #3633

when attempting to compile a query that includes "offset"; Sybase
has no straightforward "offset" feature.  fixes #2278