Remove deprecation warnings mysql5 7 20

Remove deprecation warnings mysql5 7 20

MySQL 5.7.20 now warns for use of the @tx_isolation variable; a version
check is now performed and uses @transaction_isolation instead
to prevent this warning.

For 1.1, also backport our_warn() test fixture fix from
9f0fb6c601 1.2 branch.

(cherry picked from commit 41cfe44b5e5806b3d3b13949e41dbb347bfa29e1)
Co-authored by: Mike Bayer <mike_mp@zzzcomputing.com>
Fixes: #4120
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/391
Change-Id: I4d2e04df760c5351a71dde8b32145cdc69fa6115