Add new backend for EGI Check-in

[enolfc]

Proposed changes

New backend for EGI Check-in, EGI's Identity and Access Management service
Learn more at https://www.egi.eu/service/check-in/

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[codecov]

Codecov Report

Attention: 8 lines in your changes are missing coverage. Please review.

Comparison is base (7f314b5) 77.11% compared to head (f6e1c67) 77.16%.
Report is 3 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #836      +/-   ##
==========================================
+ Coverage   77.11%   77.16%   +0.04%     
==========================================
  Files         337      339       +2     
  Lines       10262    10318      +56     
  Branches      690      694       +4     
==========================================
+ Hits         7914     7962      +48     
- Misses       2192     2200       +8     
  Partials      156      156              

Flag	Coverage Δ
unittests	77.16% <85.71%> (+0.04%)	⬆️

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Files	Coverage Δ
social_core/tests/backends/test_egi_checkin.py	100.00% <100.00%> (ø)
social_core/backends/egi_checkin.py	74.19% <74.19%> (ø)

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[enolfc]

Will change the name to make clear this is related to EGI

[nijel]

Merged, it would be great if you could also add documentation to https://github.com/python-social-auth/social-docs

[bgruening]

Thanks @enolfc !