Remove python 3.5 reference from README (#21749)
Remove python 3.5 reference from README (#21749)

Airflow 1.10.x has been EOL long enough now that we don't need to keep
sections specific to it in the main README any longer.
apache/airflow,bolkedebruin/airflow,lyft/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,bolkedebruin/airflow,apache/airflow,apache/airflow,apache/airflow,lyft/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,apache/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,nathanielvarona/airflow,cfei18/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,lyft/incubator-airflow,nathanielvarona/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow