Make sure asyncpgfallback has __slots__
Make sure asyncpgfallback has __slots__

the test suite failed to find that we started accessing
a non-existent slot "_isolation_setting" added by me in
9b779611f9, as the test suite makes use of the
AsyncAdaptFallback_asyncpg_connection subclass, which didn't
include __slots__ and therefore didn't catch that
_isolation_setting wasn't added to slots.

Fixes: #5739
Change-Id: Ibbbedc2ee0f1d1c9d91ba7898d755812deccb380

j5int/sqlalchemy,monetate/sqlalchemy,sqlalchemy/sqlalchemy,zzzeek/sqlalchemy,j5int/sqlalchemy,monetate/sqlalchemy