Fix extras naming with_social -> with-social

[pomali]

Replace underscore with hyphen to conform with Core metadata specification https://packaging.python.org/en/latest/specifications/core-metadata/#provides-extra-multiple-use

Found need for this fix because poetry "normalizes" the extras, and when you export requirements.txt from poetry it breaks the extras

[Aniket-Singla]

I believe this change was not backwards compatible for dependency specifications (eg. requirements.txt). So if people had dj-rest-auth[with_social] even for any previous version of dj-rest-auth prior to this change, I am expecting their builds to fail. But anyways should be a quickfix.