MySQL's native ENUM type supports any non-valid value being sent, and
in response will return a blank string.  A hardcoded rule to check for
"is returning the blank string" has been added to the  MySQL
implementation for ENUM so that this blank string is returned to the
application rather than being rejected as a non-valid value.  Note that
if your MySQL enum is linking values to objects, you still get the
blank string back.

Change-Id: I61f85c20293a48b0c11a31f2a19f6756c206bd20
Fixes: #3841

Change-Id: I23a6abb26bbbe3d118887d043ce761fc4572d8d2
Fixes: #3766

Change-Id: Ied4245433d0d7b469dae6e7394c4931d8405f387

Rolled back the validation rules a bit in :class:`.Enum` to allow
unknown string values to pass through, unless the flag
``validate_string=True`` is passed to the Enum; any other kind of object is
still of course rejected.  While the immediate use
is to allow comparisons to enums with LIKE, the fact that this
use exists indicates there may be more unknown-string-comparsion use
cases than we expected, which hints that perhaps there are some
unknown string-INSERT cases too.

Change-Id: I7d1d79b374a7d47966d410998f77cd19294ab7b0
Fixes: #3725

Pull request courtesy Stefan Urbanek.  fixes #1957

- make docs for isolation level more consistent between postgresql
and mysql
- move mysql autocommit tests

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

- test_query isn't assertscompiledsql