Pin pys2index >=0.1.5 in osx environment

Description

Github Actions tests for OSX are failing; this is coming from pys2index, and I noticed the environment installs an old version (0.1.4), so I am pinning it in an attempt to avert the fails; I suspect this will not fix the problem, since what the actual problem is I believe it to be the start of the incompatibility saga of M1 ARM processors vs generic intel-compiled code, but fingers crossed a simple pin will fix the barf :beer:

EDIT: the pin worked! https://github.com/ESMValGroup/ESMValTool/actions/runs/11553713844/job/32155462033?pr=3792

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ESMValGroup / ESMValTool