remove async session slots
remove async session slots

1798c3cf1c added __slots__ to the base which then
caused af0824fd79 to fail.  as we can't use an assignable
class variable with slots, remove slots from AsyncSession
which is how this class was working anyway.

Fixes: #6967
Change-Id: I4e0adab923db8e77cf748a8728e253258838e8f1

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