bug#52269: [core-updates-frozen] Some Python packages relying on .pth are broken

[Maxim Cournoyer]

Hello Guix,

This was already something Harmut noted during their review of the
site.py loader (that it should honor .pth files), but at the time I
wasn't aware of a Python package that still made use of that mechanism
and thought it was legacy.

To my dismay it seems to be used by the tool 'pdbpp', which is an
improved pdb (debugger) for Python; using core-updates-frozen I noticed
that it was no longer in use; looking at its installed files I see:

--8<---------------cut here---------------start------------->8---
pdbpp_hijack_pdb.pth
--8<---------------cut here---------------end--------------->8---

So I'm guessing that because the new loader doesn't handle .pth files
its "hijacking" technique doesn't work.

Unfortunately touching this site.py file would causes a massive rebuild
(of the whole Python world).  Hopefully this use of .pth is a rare
occurrence and can be worked around.

Thanks,

Maxim