ref #3155

events from firing for those statements which it uses internally
to detect if a table exists or not.   This is achieved using an
execution option ``skip_user_error_events`` that disables the handle
error event for the scope of that execution.   In this way, user code
that rewrites exceptions doesn't need to worry about the MySQL
dialect or other dialects that occasionally need to catch
SQLAlchemy specific exceptions.

sqlalchemy/orm, sqlalchemy/event, sqlalchemy/testing

to get all flake8 passing

ProgrammingError, not OperationalError, in modern MySQL-Python versions;
all MySQL error codes that are tested for "is disconnect" are now
checked within OperationalError and ProgrammingError regardless.
fixes #3101

on an index needed to have the same quoting for quoted names in
order to be recognized.  The fix makes the quotes optional but
also provides the old behavior for backwards compatibility with those
using the workaround.
fixes #3085

- fix a bad comparison in MySQL w/ limit/offset thing