[AIRFLOW-2849] Add flake8 to setup.py
[AIRFLOW-2849] Add flake8 to setup.py

Closes #3694 from eyaltrabelsi/AIRFLOW-2849-add-flake8-to-setup.py-dev-req-to-run-quality-check-locally

r39132/airflow,sid88in/incubator-airflow,edgarRd/incubator-airflow,malmiron/incubator-airflow,akosel/incubator-airflow,gtoonstra/airflow,adamhaney/airflow,mistercrunch/airflow,artwr/airflow,wileeam/airflow,Acehaidrey/incubator-airflow,airbnb/airflow,r39132/airflow,jgao54/airflow,DinoCow/airflow,criccomini/airflow,airbnb/airflow,wileeam/airflow,apache/airflow,edgarRd/incubator-airflow,fenglu-g/incubator-airflow,jgao54/airflow,spektom/incubator-airflow,jgao54/airflow,akosel/incubator-airflow,lyft/incubator-airflow,cfei18/incubator-airflow,cfei18/incubator-airflow,nathanielvarona/airflow,fenglu-g/incubator-airflow,wooga/airflow,artwr/airflow,apache/airflow,apache/incubator-airflow,sid88in/incubator-airflow,danielvdende/incubator-airflow,criccomini/airflow,mistercrunch/airflow,danielvdende/incubator-airflow,airbnb/airflow,mrkm4ntr/incubator-airflow,DinoCow/airflow,dhuang/incubator-airflow,airbnb/airflow,andyxhadji/incubator-airflow,Acehaidrey/incubator-airflow,andyxhadji/incubator-airflow,criccomini/airflow,apache/incubator-airflow,sekikn/incubator-airflow,mtagle/airflow,wileeam/airflow,malmiron/incubator-airflow,wileeam/airflow,bolkedebruin/airflow,Acehaidrey/incubator-airflow,adamhaney/airflow,cfei18/incubator-airflow,dhuang/incubator-airflow,gtoonstra/airflow,Fokko/incubator-airflow,artwr/airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,apache/incubator-airflow,sekikn/incubator-airflow,edgarRd/incubator-airflow,nathanielvarona/airflow,adamhaney/airflow,andyxhadji/incubator-airflow,nathanielvarona/airflow,artwr/airflow,sid88in/incubator-airflow,DinoCow/airflow,sekikn/incubator-airflow,apache/airflow,Fokko/incubator-airflow,bolkedebruin/airflow,gtoonstra/airflow,lyft/incubator-airflow,Fokko/incubator-airflow,malmiron/incubator-airflow,mtagle/airflow,mrkm4ntr/incubator-airflow,wooga/airflow,spektom/incubator-airflow,fenglu-g/incubator-airflow,danielvdende/incubator-airflow,nathanielvarona/airflow,edgarRd/incubator-airflow,cfei18/incubator-airflow,fenglu-g/incubator-airflow,nathanielvarona/airflow,lyft/incubator-airflow,r39132/airflow,mrkm4ntr/incubator-airflow,mtagle/airflow,wooga/airflow,spektom/incubator-airflow,bolkedebruin/airflow,sid88in/incubator-airflow,wooga/airflow,lyft/incubator-airflow,akosel/incubator-airflow,mrkm4ntr/incubator-airflow,r39132/airflow,cfei18/incubator-airflow,apache/airflow,gtoonstra/airflow,mtagle/airflow,Acehaidrey/incubator-airflow,andyxhadji/incubator-airflow,adamhaney/airflow,danielvdende/incubator-airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,criccomini/airflow,spektom/incubator-airflow,apache/airflow,dhuang/incubator-airflow,Fokko/incubator-airflow,nathanielvarona/airflow,apache/incubator-airflow,mistercrunch/airflow,malmiron/incubator-airflow,danielvdende/incubator-airflow,mistercrunch/airflow,sekikn/incubator-airflow,akosel/incubator-airflow,apache/airflow,bolkedebruin/airflow,jgao54/airflow,Acehaidrey/incubator-airflow,DinoCow/airflow,bolkedebruin/airflow