datahub: Stick to numpy 1.18.5
datahub: Stick to numpy 1.18.5

A bunch of libraries - particularly tensorflow - don't
support 1.19 yet

berkeley-dsep-infra/datahub,ryanlovett/datahub,ryanlovett/datahub,ryanlovett/datahub,berkeley-dsep-infra/datahub,berkeley-dsep-infra/datahub