Apply minor update to async_orm example
Apply minor update to async_orm example

- Fixed import to avoid MovedIn20Warning.

- Separated drop_all() and create_all() into separate
context managers to avoid dropping and creating the
same table within the same transaction. Apparently
some databases (e.g., CockroachDB) don't allow that.

Change-Id: Id26d7d719871a75ffb78c6af589658666802fb2f

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