Replace django-rest-auth with dj-rest-auth

[aptiko]

Checklist:

• [ ] All tests are passing
• [ ] I have added any necessary documentation and/or release notes
• [ ] I have followed the commit message guidelines and the "before submitting a pull request" guidelines
• [ ] If I introduced (or abolished) any third-party library, I will write a short paragraph in this PR according to the guidelines
• [ ] I have removed all obsoleted code

django-rest-auth is unmaintained. It has been forked at dj-rest-auth, which seems to be the most popular replacement and needs minimal changes to the code.

[codecov[bot]]

Codecov Report

Merging #444 (24801f1) into master (228d62d) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #444   +/-   ##
=======================================
  Coverage   97.64%   97.64%           
=======================================
  Files          19       19           
  Lines        1486     1486           
=======================================
  Hits         1451     1451           
  Misses         35       35           

Impacted Files	Coverage Δ
enhydris/urls.py	100.00% <ø> (ø)
enhydris/api/serializers_captcha.py	94.44% <100.00%> (ø)
enhydris/api/urls.py	100.00% <100.00%> (ø)

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