[AIRFLOW-5325] Default python version if no python3 on path is 3.5
[AIRFLOW-5325] Default python version if no python3 on path is 3.5

(cherry picked from commit 6950a2b1cd42b31be75c5ec0bf3627f30d791e21)

owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow,owlabs/incubator-airflow