[AIRFLOW-2713] Rename async variable in setup.py for Python 3.7.0 compatibility
[AIRFLOW-2713] Rename async variable in setup.py for Python 3.7.0 compatibility

Closes #3561 from Perados/rename-async-to-
async_packages-in-setup

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