[AIRFLOW-2713] Rename async variable in setup.py for Python 3.7.0 compatibility
[AIRFLOW-2713] Rename async variable in setup.py for Python 3.7.0 compatibility

Closes #3561 from Perados/rename-async-to-
async_packages-in-setup

lxneng/incubator-airflow,lxneng/incubator-airflow,lxneng/incubator-airflow,lxneng/incubator-airflow