update rtd-pip-requirements to try to fix doc build
update rtd-pip-requirements to try to fix doc build

jpinedaf/pyspeckit,vlas-sokolov/pyspeckit,e-koch/pyspeckit,jpinedaf/pyspeckit,mikelum/pyspeckit,e-koch/pyspeckit,pyspeckit/pyspeckit,keflavich/pyspeckit,keflavich/pyspeckit,vlas-sokolov/pyspeckit,low-sky/pyspeckit,pyspeckit/pyspeckit,mikelum/pyspeckit,allisony/pyspeckit,allisony/pyspeckit,low-sky/pyspeckit