Skip registering build_docs if sphinx missing some of its dependencies.
Skip registering build_docs if sphinx missing some of its dependencies.

dpshelio/astropy-helpers,bsipocz/astropy-helpers,astropy/astropy-helpers,Cadair/astropy-helpers,bsipocz/astropy-helpers,bsipocz/astropy-helpers,larrybradley/astropy-helpers,dpshelio/astropy-helpers,astropy/astropy-helpers,larrybradley/astropy-helpers,Cadair/astropy-helpers,larrybradley/astropy-helpers