AppVeyor: disable installing graphviz due to issues with the graphviz server
AppVeyor: disable installing graphviz due to issues with the graphviz server
Cadair/astropy-helpers,astropy/astropy-helpers,astropy/astropy-helpers,Cadair/astropy-helpers,larrybradley/astropy-helpers,larrybradley/astropy-helpers,dpshelio/astropy-helpers,bsipocz/astropy-helpers,bsipocz/astropy-helpers,larrybradley/astropy-helpers,bsipocz/astropy-helpers,dpshelio/astropy-helpers