feat(backends): implement a generic CAS backend

[sinavir]

This commit introduces support for the CAS protocol using python-cas library.

It is tested using zulip (for the client) and django-cas-server (for the cas server).

I'm not very confident about how to write the test for this. I'm interested to know if you recommend any code or documentation interesting to get that task done

[nijel]

Thanks for the pull requests. The tests typically mock the authentication API provider, check existing tests for inspiration. Documentation should be added to https://github.com/python-social-auth/social-docs/.

[nijel]

@codex review

[nijel]

This should probably go to an extra, not be part of core deps like Google OneTap or Shopify.

[codecov]

Codecov Report

❌ Patch coverage is 0% with 30 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.00%. Comparing base (c0b4ecc) to head (da7fdf8).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
social_core/backends/cas_generic.py	0.00%	30 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1986      +/-   ##
==========================================
- Coverage   89.19%   89.00%   -0.20%     
==========================================
  Files         345      346       +1     
  Lines       13926    13956      +30     
  Branches      698      700       +2     
==========================================
  Hits        12421    12421              
- Misses       1262     1292      +30     
  Partials      243      243              

Flag	Coverage Δ
unittests	89.00% <0.00%> (-0.20%)	⬇️

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