Add an "htmlview" target to open docs after building them.
Add an "htmlview" target to open docs after building them.

This is helpful if you build and test documentation updates in
your development env.

Copied from CPython's Doc/Makefile.

gtrdotmcs/gunicorn,ccl0326/gunicorn,mvaled/gunicorn,gtrdotmcs/gunicorn,tempbottle/gunicorn,z-fork/gunicorn,jamesblunt/gunicorn,GitHublong/gunicorn,mvaled/gunicorn,keakon/gunicorn,zhoucen/gunicorn,mvaled/gunicorn,harrisonfeng/gunicorn,prezi/gunicorn,elelianghh/gunicorn,MrKiven/gunicorn,gtrdotmcs/gunicorn,ccl0326/gunicorn,zhoucen/gunicorn,zhoucen/gunicorn,malept/gunicorn,jamesblunt/gunicorn,malept/gunicorn,jamesblunt/gunicorn,beni55/gunicorn,tejasmanohar/gunicorn,WSDC-NITWarangal/gunicorn,ephes/gunicorn,malept/gunicorn,ccl0326/gunicorn,prezi/gunicorn,prezi/gunicorn