Added a tox job for running flake8 under Python 3
Added a tox job for running flake8 under Python 3

Change-Id: I04a78cc5e3613d1d765d716633bca297bf7207b6

takeshineshiro/swift,swiftstack/swift,openstack/swift,Akanoa/swift,psachin/swift,scality/ScalitySproxydSwift,hbhdytf/mac2,redhat-openstack/swift,dencaval/swift,dencaval/swift,clayg/swift,bradleypj823/swift,Em-Pan/swift,wenhuizhang/swift,aerwin3/swift,bradleypj823/swift,matthewoliver/swift,aerwin3/swift,hbhdytf/mac2,Khushbu27/Tutorial,mjwtom/swift,williamthegrey/swift,hurricanerix/swift,tipabu/swift,hbhdytf/mac2,nadeemsyed/swift,iostackproject/IO-Bandwidth-Differentiation,psachin/swift,bouncestorage/swift,mjwtom/swift,thiagodasilva/swift,Em-Pan/swift,daasbank/swift,nadeemsyed/swift,zackmdavis/swift,redbo/swift,prashanthpai/swift,IPVL/swift-kilo,larsbutler/swift,smerritt/swift,levythu/swift,gold3bear/swift,bouncestorage/swift,shibaniahegde/OpenStak_swift,revoer/keystone-8.0.0,matthewoliver/swift,takeshineshiro/swift,rackerlabs/swift,hbhdytf/mac2,hurricanerix/swift,larsbutler/swift,wenhuizhang/swift,hbhdytf/mac,clayg/swift,levythu/swift,eatbyte/Swift,anishnarang/gswift,hbhdytf/mac,tipabu/swift,notmyname/swift,Seagate/swift,zackmdavis/swift,shibaniahegde/OpenStak_swift,clayg/swift,notmyname/swift,bkolli/swift,hurricanerix/swift,notmyname/swift,openstack/swift,IPVL/swift-kilo,swiftstack/swift,iostackproject/IO-Bandwidth-Differentiation,williamthegrey/swift,smerritt/swift,Khushbu27/Tutorial,openstack/swift,rackerlabs/swift,nadeemsyed/swift,nadeemsyed/swift,AfonsoFGarcia/swift,tipabu/swift,matthewoliver/swift,psachin/swift,smerritt/swift,daasbank/swift,hurricanerix/swift,scality/ScalitySproxydSwift,mjzmjz/swift,notmyname/swift,psachin/swift,anishnarang/gswift,swiftstack/swift,tipabu/swift,Seagate/swift,smerritt/swift,maginatics/swift,AfonsoFGarcia/swift,matthewoliver/swift,bkolli/swift,maginatics/swift,redhat-openstack/swift,mjzmjz/swift,sarvesh-ranjan/swift,gold3bear/swift,revoer/keystone-8.0.0,thiagodasilva/swift,dpgoetz/swift,eatbyte/Swift,openstack/swift,prashanthpai/swift,sarvesh-ranjan/swift,clayg/swift,Akanoa/swift,redbo/swift,dpgoetz/swift