add test to ensure that 'pip check' doesn't return any broken requirements for installed Python packages in compat layer

[boegel]

Works fine for 2021.06 compat layer:

[kehoste@login1 ~]$ /cvmfs/pilot.eessi-hpc.org/2021.06/compat/linux/x86_64/startprefix
Entering Gentoo Prefix /cvmfs/pilot.eessi-hpc.org/2021.06/compat/linux/x86_64
bash: module: command not found
kehoste@login1 ~ $ pip check
No broken requirements found.

But in the draft 2021.12 compat layer (built with playbook from #140):

eessi@ip-172-31-25-60 ~/software-layer/software-layer $ which pip
/cvmfs/pilot.eessi-hpc.org/2021.12/compat/linux/aarch64/usr/bin/pip
eessi@ip-172-31-25-60 ~/software-layer/software-layer $ pip check
gitpython 3.1.24 requires typing-extensions, which is not installed.

This is most likely an upstream issue in Gentoo (missing dependency)...

[bedroge]

Opened a Gentoo bug report, as it looked like an issue with the ebuild file: https://bugs.gentoo.org/827882

[bedroge]

Actually it was a bug in the requirements.txt of GitPython itself, as they no longer need it:
https://github.com/gitpython-developers/GitPython/blob/3.1.24/git/types.py#L10

Gentoo quickly fixed it by adding a workaround in the new ebuild file:
gentoo/gentoo@4af18f3

[boegel]

I think we should add pip check to our set of tests for compat layer, so I'll re-open this.

[bedroge]

Oh, sorry, that makes sense, indeed. I overlooked that making a test was part of the issue...

[boegel]

See also #152 (which is a duplicate, I guess)