[AIRFLOW-1613] make mysql_to_gcs_operator py3 compatible
[AIRFLOW-1613] make mysql_to_gcs_operator py3 compatible

Uses `__future__.unicode_literals` and replaces calling `json.dumps`
with `json.dump` followed by `tmp_file_handle.write` to write json lines
to the ndjson file. When using python3, `json.dump` will return a
unicode string instead of a byte string, therefore we encode the unicode
string to `utf-8` which is compatible with bigquery (see:
https://cloud.google.com/bigquery/docs/loading-data#loading_encoded_data).

Acehaidrey/incubator-airflow,mtagle/airflow,r39132/airflow,MortalViews/incubator-airflow,KL-WLCR/incubator-airflow,dhuang/incubator-airflow,danielvdende/incubator-airflow,danielvdende/incubator-airflow,mtagle/airflow,airbnb/airflow,artwr/airflow,apache/incubator-airflow,sid88in/incubator-airflow,jfantom/incubator-airflow,wileeam/airflow,mrkm4ntr/incubator-airflow,Acehaidrey/incubator-airflow,OpringaoDoTurno/airflow,sid88in/incubator-airflow,KL-WLCR/incubator-airflow,spektom/incubator-airflow,RealImpactAnalytics/airflow,artwr/airflow,adamhaney/airflow,wileeam/airflow,sekikn/incubator-airflow,wooga/airflow,nathanielvarona/airflow,gtoonstra/airflow,andyxhadji/incubator-airflow,lxneng/incubator-airflow,mrkm4ntr/incubator-airflow,artwr/airflow,edgarRd/incubator-airflow,mtagle/airflow,sekikn/incubator-airflow,mrkm4ntr/incubator-airflow,KL-WLCR/incubator-airflow,adamhaney/airflow,OpringaoDoTurno/airflow,malmiron/incubator-airflow,cfei18/incubator-airflow,zack3241/incubator-airflow,edgarRd/incubator-airflow,yati-sagade/incubator-airflow,CloverHealth/airflow,sergiohgz/incubator-airflow,lyft/incubator-airflow,yati-sagade/incubator-airflow,MortalViews/incubator-airflow,nathanielvarona/airflow,jfantom/incubator-airflow,DinoCow/airflow,airbnb/airflow,apache/airflow,wndhydrnt/airflow,jfantom/incubator-airflow,mrkm4ntr/incubator-airflow,fenglu-g/incubator-airflow,danielvdende/incubator-airflow,wolfier/incubator-airflow,fenglu-g/incubator-airflow,danielvdende/incubator-airflow,zack3241/incubator-airflow,nathanielvarona/airflow,malmiron/incubator-airflow,malmiron/incubator-airflow,yk5/incubator-airflow,apache/airflow,nathanielvarona/airflow,wolfier/incubator-airflow,wolfier/incubator-airflow,yati-sagade/incubator-airflow,r39132/airflow,wileeam/airflow,subodhchhabra/airflow,KL-WLCR/incubator-airflow,sid88in/incubator-airflow,nathanielvarona/airflow,apache/airflow,spektom/incubator-airflow,yati-sagade/incubator-airflow,zack3241/incubator-airflow,owlabs/incubator-airflow,bolkedebruin/airflow,MortalViews/incubator-airflow,subodhchhabra/airflow,mistercrunch/airflow,wooga/airflow,lxneng/incubator-airflow,lxneng/incubator-airflow,yk5/incubator-airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,owlabs/incubator-airflow,apache/incubator-airflow,DinoCow/airflow,RealImpactAnalytics/airflow,lyft/incubator-airflow,subodhchhabra/airflow,andyxhadji/incubator-airflow,cfei18/incubator-airflow,RealImpactAnalytics/airflow,zack3241/incubator-airflow,dhuang/incubator-airflow,lyft/incubator-airflow,criccomini/airflow,Acehaidrey/incubator-airflow,cfei18/incubator-airflow,Fokko/incubator-airflow,fenglu-g/incubator-airflow,criccomini/airflow,spektom/incubator-airflow,gtoonstra/airflow,edgarRd/incubator-airflow,MortalViews/incubator-airflow,CloverHealth/airflow,wndhydrnt/airflow,r39132/airflow,apache/incubator-airflow,sid88in/incubator-airflow,DinoCow/airflow,spektom/incubator-airflow,lyft/incubator-airflow,mtagle/airflow,cfei18/incubator-airflow,akosel/incubator-airflow,nathanielvarona/airflow,mistercrunch/airflow,yk5/incubator-airflow,gtoonstra/airflow,sekikn/incubator-airflow,sergiohgz/incubator-airflow,Fokko/incubator-airflow,apache/airflow,mistercrunch/airflow,Acehaidrey/incubator-airflow,bolkedebruin/airflow,andyxhadji/incubator-airflow,wooga/airflow,sergiohgz/incubator-airflow,wndhydrnt/airflow,RealImpactAnalytics/airflow,akosel/incubator-airflow,OpringaoDoTurno/airflow,yk5/incubator-airflow,criccomini/airflow,Tagar/incubator-airflow,cfei18/incubator-airflow,wndhydrnt/airflow,Acehaidrey/incubator-airflow,fenglu-g/incubator-airflow,jgao54/airflow,lxneng/incubator-airflow,adamhaney/airflow,akosel/incubator-airflow,Tagar/incubator-airflow,dhuang/incubator-airflow,wolfier/incubator-airflow,sekikn/incubator-airflow,danielvdende/incubator-airflow,dhuang/incubator-airflow,r39132/airflow,malmiron/incubator-airflow,Tagar/incubator-airflow,wooga/airflow,subodhchhabra/airflow,wileeam/airflow,owlabs/incubator-airflow,jgao54/airflow,airbnb/airflow,mistercrunch/airflow,Fokko/incubator-airflow,owlabs/incubator-airflow,bolkedebruin/airflow,artwr/airflow,cfei18/incubator-airflow,akosel/incubator-airflow,CloverHealth/airflow,jgao54/airflow,jgao54/airflow,edgarRd/incubator-airflow,CloverHealth/airflow,apache/incubator-airflow,Tagar/incubator-airflow,adamhaney/airflow,gtoonstra/airflow,sergiohgz/incubator-airflow,bolkedebruin/airflow,DinoCow/airflow,OpringaoDoTurno/airflow,jfantom/incubator-airflow,airbnb/airflow,criccomini/airflow,apache/airflow,danielvdende/incubator-airflow,andyxhadji/incubator-airflow,Fokko/incubator-airflow,apache/airflow