Pin scipy version since the most updated scipy is not compatible with lalinference postprocess.

The most updated version of scipy is not compatible with lalinference postparocess and prevents us from testing lalinference workflow with playground events, e.g., https://gracedb-playground.ligo.org/apiweb/superevents/S190606t/files/resultspage-25716-0.err.log,0 .

@leo-singer @roberto.depietri Could you please take a look at this? Without this patch, we can not test the recent version of automatic PE.