tools/check-py3: Speed up and add --find-fixers.
tools/check-py3: Speed up and add --find-fixers.

Now tools/check-py3 will by default run all fixers together.  This is
quicker but doesn't indicate which fixers caused the failure.  The
newly added option --find-fixers falls back to the old way of checking
each fixer separately if the quick check fails.

Fixes #710.

christi3k/zulip,cosmicAsymmetry/zulip,krtkmj/zulip,synicalsyntax/zulip,souravbadami/zulip,Vallher/zulip,dhcrzf/zulip,ryanbackman/zulip,rht/zulip,verma-varsha/zulip,amanharitsh123/zulip,sonali0901/zulip,reyha/zulip,vabs22/zulip,krtkmj/zulip,jrowan/zulip,amyliu345/zulip,brockwhittaker/zulip,peiwei/zulip,peiwei/zulip,Jianchun1/zulip,Juanvulcano/zulip,ahmadassaf/zulip,shubhamdhama/zulip,rht/zulip,rishig/zulip,aakash-cr7/zulip,jphilipsen05/zulip,ryanbackman/zulip,rishig/zulip,joyhchen/zulip,umkay/zulip,dattatreya303/zulip,cosmicAsymmetry/zulip,paxapy/zulip,joyhchen/zulip,AZtheAsian/zulip,calvinleenyc/zulip,synicalsyntax/zulip,andersk/zulip,Juanvulcano/zulip,Galexrt/zulip,christi3k/zulip,Diptanshu8/zulip,isht3/zulip,samatdav/zulip,Frouk/zulip,timabbott/zulip,peiwei/zulip,mahim97/zulip,punchagan/zulip,TigorC/zulip,andersk/zulip,brainwane/zulip,verma-varsha/zulip,kou/zulip,Diptanshu8/zulip,christi3k/zulip,reyha/zulip,ahmadassaf/zulip,sup95/zulip,mohsenSy/zulip,tommyip/zulip,mohsenSy/zulip,Vallher/zulip,andersk/zulip,rht/zulip,sonali0901/zulip,sup95/zulip,jrowan/zulip,hackerkid/zulip,brockwhittaker/zulip,JPJPJPOPOP/zulip,dawran6/zulip,souravbadami/zulip,rishig/zulip,showell/zulip,Diptanshu8/zulip,umkay/zulip,niftynei/zulip,arpith/zulip,arpith/zulip,KingxBanana/zulip,arpith/zulip,timabbott/zulip,krtkmj/zulip,hackerkid/zulip,ahmadassaf/zulip,vabs22/zulip,Vallher/zulip,amanharitsh123/zulip,amyliu345/zulip,samatdav/zulip,showell/zulip,zulip/zulip,mohsenSy/zulip,umkay/zulip,souravbadami/zulip,zulip/zulip,Juanvulcano/zulip,jainayush975/zulip,andersk/zulip,tommyip/zulip,zacps/zulip,aakash-cr7/zulip,amanharitsh123/zulip,brockwhittaker/zulip,j831/zulip,j831/zulip,umkay/zulip,reyha/zulip,dhcrzf/zulip,calvinleenyc/zulip,showell/zulip,paxapy/zulip,peguin40/zulip,andersk/zulip,Galexrt/zulip,Jianchun1/zulip,cosmicAsymmetry/zulip,jainayush975/zulip,PhilSk/zulip,zacps/zulip,reyha/zulip,souravbadami/zulip,krtkmj/zulip,Vallher/zulip,dawran6/zulip,peiwei/zulip,sonali0901/zulip,PhilSk/zulip,shubhamdhama/zulip,paxapy/zulip,vaidap/zulip,mahim97/zulip,JPJPJPOPOP/zulip,PhilSk/zulip,synicalsyntax/zulip,vikas-parashar/zulip,isht3/zulip,brockwhittaker/zulip,eeshangarg/zulip,eeshangarg/zulip,peguin40/zulip,grave-w-grave/zulip,sharmaeklavya2/zulip,brainwane/zulip,eeshangarg/zulip,punchagan/zulip,krtkmj/zulip,susansls/zulip,amyliu345/zulip,jackrzhang/zulip,vikas-parashar/zulip,samatdav/zulip,Frouk/zulip,dhcrzf/zulip,mohsenSy/zulip,synicalsyntax/zulip,dattatreya303/zulip,zacps/zulip,Galexrt/zulip,umkay/zulip,niftynei/zulip,Galexrt/zulip,shubhamdhama/zulip,kou/zulip,amanharitsh123/zulip,showell/zulip,punchagan/zulip,susansls/zulip,niftynei/zulip,verma-varsha/zulip,reyha/zulip,amyliu345/zulip,isht3/zulip,showell/zulip,hackerkid/zulip,calvinleenyc/zulip,peguin40/zulip,vaidap/zulip,krtkmj/zulip,grave-w-grave/zulip,dawran6/zulip,tommyip/zulip,sonali0901/zulip,jrowan/zulip,mahim97/zulip,j831/zulip,Diptanshu8/zulip,AZtheAsian/zulip,jainayush975/zulip,verma-varsha/zulip,dawran6/zulip,KingxBanana/zulip,peguin40/zulip,calvinleenyc/zulip,samatdav/zulip,sonali0901/zulip,arpith/zulip,vaidap/zulip,ahmadassaf/zulip,showell/zulip,brainwane/zulip,umkay/zulip,isht3/zulip,zulip/zulip,vaidap/zulip,verma-varsha/zulip,susansls/zulip,dhcrzf/zulip,rht/zulip,joyhchen/zulip,vaidap/zulip,brockwhittaker/zulip,tommyip/zulip,dhcrzf/zulip,Frouk/zulip,vabs22/zulip,sharmaeklavya2/zulip,synicalsyntax/zulip,paxapy/zulip,j831/zulip,peguin40/zulip,sup95/zulip,shubhamdhama/zulip,arpith/zulip,calvinleenyc/zulip,timabbott/zulip,Frouk/zulip,zacps/zulip,jackrzhang/zulip,jphilipsen05/zulip,ryanbackman/zulip,joyhchen/zulip,dhcrzf/zulip,kou/zulip,christi3k/zulip,amyliu345/zulip,mahim97/zulip,KingxBanana/zulip,rishig/zulip,niftynei/zulip,blaze225/zulip,SmartPeople/zulip,tommyip/zulip,zacps/zulip,blaze225/zulip,jrowan/zulip,kou/zulip,mohsenSy/zulip,aakash-cr7/zulip,jrowan/zulip,punchagan/zulip,Juanvulcano/zulip,tommyip/zulip,jackrzhang/zulip,TigorC/zulip,aakash-cr7/zulip,shubhamdhama/zulip,brainwane/zulip,PhilSk/zulip,eeshangarg/zulip,vabs22/zulip,JPJPJPOPOP/zulip,SmartPeople/zulip,peiwei/zulip,isht3/zulip,jackrzhang/zulip,reyha/zulip,Vallher/zulip,cosmicAsymmetry/zulip,SmartPeople/zulip,TigorC/zulip,tommyip/zulip,isht3/zulip,Frouk/zulip,Galexrt/zulip,eeshangarg/zulip,JPJPJPOPOP/zulip,zacps/zulip,sharmaeklavya2/zulip,andersk/zulip,samatdav/zulip,SmartPeople/zulip,jphilipsen05/zulip,sharmaeklavya2/zulip,Jianchun1/zulip,SmartPeople/zulip,brockwhittaker/zulip,kou/zulip,zulip/zulip,showell/zulip,kou/zulip,timabbott/zulip,SmartPeople/zulip,synicalsyntax/zulip,niftynei/zulip,hackerkid/zulip,blaze225/zulip,hackerkid/zulip,grave-w-grave/zulip,ahmadassaf/zulip,vabs22/zulip,sonali0901/zulip,cosmicAsymmetry/zulip,rishig/zulip,jainayush975/zulip,peiwei/zulip,shubhamdhama/zulip,amyliu345/zulip,timabbott/zulip,susansls/zulip,jphilipsen05/zulip,j831/zulip,ryanbackman/zulip,grave-w-grave/zulip,Diptanshu8/zulip,brainwane/zulip,krtkmj/zulip,j831/zulip,zulip/zulip,joyhchen/zulip,arpith/zulip,dattatreya303/zulip,jackrzhang/zulip,jainayush975/zulip,ahmadassaf/zulip,AZtheAsian/zulip,punchagan/zulip,blaze225/zulip,calvinleenyc/zulip,dawran6/zulip,cosmicAsymmetry/zulip,TigorC/zulip,TigorC/zulip,TigorC/zulip,jainayush975/zulip,susansls/zulip,KingxBanana/zulip,vikas-parashar/zulip,jackrzhang/zulip,peguin40/zulip,vikas-parashar/zulip,Frouk/zulip,susansls/zulip,JPJPJPOPOP/zulip,vabs22/zulip,AZtheAsian/zulip,mahim97/zulip,sup95/zulip,Juanvulcano/zulip,eeshangarg/zulip,punchagan/zulip,jphilipsen05/zulip,ahmadassaf/zulip,aakash-cr7/zulip,souravbadami/zulip,verma-varsha/zulip,hackerkid/zulip,Diptanshu8/zulip,grave-w-grave/zulip,amanharitsh123/zulip,Jianchun1/zulip,rishig/zulip,Galexrt/zulip,aakash-cr7/zulip,jrowan/zulip,hackerkid/zulip,dattatreya303/zulip,peiwei/zulip,Vallher/zulip,PhilSk/zulip,christi3k/zulip,timabbott/zulip,KingxBanana/zulip,niftynei/zulip,timabbott/zulip,souravbadami/zulip,kou/zulip,Frouk/zulip,ryanbackman/zulip,synicalsyntax/zulip,AZtheAsian/zulip,eeshangarg/zulip,jphilipsen05/zulip,dhcrzf/zulip,Vallher/zulip,rishig/zulip,blaze225/zulip,ryanbackman/zulip,JPJPJPOPOP/zulip,blaze225/zulip,mahim97/zulip,brainwane/zulip,AZtheAsian/zulip,Galexrt/zulip,christi3k/zulip,zulip/zulip,andersk/zulip,samatdav/zulip,sharmaeklavya2/zulip,amanharitsh123/zulip,paxapy/zulip,sharmaeklavya2/zulip,rht/zulip,shubhamdhama/zulip,brainwane/zulip,sup95/zulip,Jianchun1/zulip,grave-w-grave/zulip,sup95/zulip,dawran6/zulip,vaidap/zulip,Jianchun1/zulip,joyhchen/zulip,KingxBanana/zulip,dattatreya303/zulip,vikas-parashar/zulip,jackrzhang/zulip,umkay/zulip,PhilSk/zulip,Juanvulcano/zulip,paxapy/zulip,dattatreya303/zulip,mohsenSy/zulip,rht/zulip,rht/zulip,vikas-parashar/zulip,punchagan/zulip,zulip/zulip