simplify tox again now that we can exclude tests more easily

rules, better message formatting

sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

to get all flake8 passing

- implement for SQL server, use window functions when simple limit/offset not available

Found using: https://github.com/intgr/topy

it for DELETE would fail to target the correct row for DELETE.
Then to compound matters, basic "number of rows matched" checks were
not being performed.  Both issues are fixed, however note that the
"rows matched" check requires so-called "sane multi-row count"
functionality; the DBAPI's executemany() method must count up the
rows matched by individual statements and SQLAlchemy's dialect must
mark this feature as supported, currently applies to some mysql dialects,
psycopg2, sqlite only. fixes #3006
- Enabled "sane multi-row count" checking for the psycopg2 DBAPI, as
this seems to be supported as of psycopg2 2.0.9.

"supports unicode statements" flag is now False, so that SQLAlchemy
will encode the *SQL string* (note: *not* the parameters)
to bytes before sending to the database.  This seems to allow
all unicode-related tests to pass for mysql-connector, including those
that use non-ascii table/column names, as well as some tests for the
TEXT type using unicode under cursor.executemany().
- other mysql-connector fixes; latest version seems to do better on
function call counts

SQLA internal tests outside; plus things like savepoints, twophase