[AIRFLOW-3190] Make flake8 compliant
[AIRFLOW-3190] Make flake8 compliant

One voilation that slipped in by PR that didn't rebase onto
latest master

apache/incubator-airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,sekikn/incubator-airflow,mrkm4ntr/incubator-airflow,airbnb/airflow,malmiron/incubator-airflow,artwr/airflow,cfei18/incubator-airflow,Acehaidrey/incubator-airflow,apache/airflow,malmiron/incubator-airflow,r39132/airflow,apache/incubator-airflow,cfei18/incubator-airflow,spektom/incubator-airflow,mrkm4ntr/incubator-airflow,wileeam/airflow,r39132/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,artwr/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,mistercrunch/airflow,lyft/incubator-airflow,nathanielvarona/airflow,DinoCow/airflow,nathanielvarona/airflow,mtagle/airflow,Fokko/incubator-airflow,adamhaney/airflow,artwr/airflow,jgao54/airflow,danielvdende/incubator-airflow,mtagle/airflow,bolkedebruin/airflow,jgao54/airflow,nathanielvarona/airflow,r39132/airflow,lyft/incubator-airflow,dhuang/incubator-airflow,Fokko/incubator-airflow,mrkm4ntr/incubator-airflow,Acehaidrey/incubator-airflow,fenglu-g/incubator-airflow,mtagle/airflow,r39132/airflow,DinoCow/airflow,cfei18/incubator-airflow,fenglu-g/incubator-airflow,malmiron/incubator-airflow,apache/airflow,dhuang/incubator-airflow,sekikn/incubator-airflow,spektom/incubator-airflow,apache/incubator-airflow,apache/airflow,cfei18/incubator-airflow,wooga/airflow,dhuang/incubator-airflow,mrkm4ntr/incubator-airflow,DinoCow/airflow,apache/incubator-airflow,nathanielvarona/airflow,sekikn/incubator-airflow,apache/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,airbnb/airflow,adamhaney/airflow,spektom/incubator-airflow,jgao54/airflow,fenglu-g/incubator-airflow,danielvdende/incubator-airflow,lyft/incubator-airflow,mistercrunch/airflow,DinoCow/airflow,danielvdende/incubator-airflow,cfei18/incubator-airflow,jgao54/airflow,apache/airflow,spektom/incubator-airflow,bolkedebruin/airflow,dhuang/incubator-airflow,wooga/airflow,airbnb/airflow,adamhaney/airflow,sekikn/incubator-airflow,airbnb/airflow,Fokko/incubator-airflow,nathanielvarona/airflow,malmiron/incubator-airflow,wileeam/airflow,bolkedebruin/airflow,apache/airflow,cfei18/incubator-airflow,artwr/airflow,Fokko/incubator-airflow,danielvdende/incubator-airflow,adamhaney/airflow,wileeam/airflow,wooga/airflow,mtagle/airflow,mistercrunch/airflow,danielvdende/incubator-airflow,fenglu-g/incubator-airflow,wooga/airflow,wileeam/airflow