lxml & xmlsec libxml2 library version mismatch

[nijel]

Expected behaviour

#956 introduced Python 3.13 to CI and it should work.

Actual behaviour

It does not because xmlsec wheels for 3.13 are not available and the locally built module is incompatible with the lxml wheel.

What are the steps to reproduce this issue?

See the CI outcome: https://github.com/python-social-auth/social-core/actions/runs/12633678860/job/35199792957?pr=956

Any logs, error output, etc?

ERROR social_core/tests/backends/test_saml.py - xmlsec.InternalError: (-1, 'lxml & xmlsec libxml2 library version mismatch')

Any other comments?

This is the really annoying part of using xmlsec. It has to be built with the exactly same version of libxml as the lxml module. The lxml module embeds one, so if you build xmlsec locally and use lxml wheel, you end up with mixed up versions.

This affects our SAML users as well, it makes it more tricky to use the library.

Possible approach could be to bring back workaround removed in #906 (it has to be extended to ensure xmlsec wheels are also not used because meanwhile these are available as well).