[AIRFLOW-3731] Constrain mysqlclient to <1.4 (#4558)
[AIRFLOW-3731] Constrain mysqlclient to <1.4 (#4558)

To maintain Python2 compatibility
owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow