dont rely on default_isolation_level to restore engine-level iso

dont rely on default_isolation_level to restore engine-level iso

Fixed critical issue where setting
:paramref:`_sa.create_engine.isolation_level` to ``AUTOCOMMIT`` (as opposed
to using the :meth:`_engine.Engine.execution_options` method) would fail to
restore "autocommit" to a pooled connection if an alternate isolation level
were temporarily selected using
:paramref:`_engine.Connection.execution_options.isolation_level`.

Fixes: #10147
Change-Id: Ia22c15ab460cbbf8f6a731660874ace10df7c6a6