Add support for client certs to the Python API.
Add support for client certs to the Python API.

This adds support for passing a client cert (in the format expected by the
`requests` library) to the `Client` constructor, as well as for specifying
one on the command line or in .zuliprc (through new `client_cert` and
`client_cert_key` options).

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