python: Pin fixtures dep to v4.0.0
python: Pin fixtures dep to v4.0.0

Upstream "fixtures" python package is currently half-pushed or otherwise
broken.

Change-Id: I7059ed836d7a52c5ba587ad73cc5e4b21cf84887
GitOrigin-RevId: 63714a34c3ec057d8264f1fa00da9e97e6cb7d42

AmbiML/sparrow-scripts-full,AmbiML/sparrow-scripts-full