Remove duplicate OAuthLibMixin from base classes

[fdemmer]

Description of the Change

The generic view classes ProtectedResourceView and ClientProtectedResourceView inherit from ProtectedResourceMixin and ClientProtectedResourceMixin respectively, which both inherit OAuthLibMixin already.

It seems redundant to me to have the views inherit OAuthLibMixin again.

If there is no reason I am missing, this PR fixes that.

Checklist

• PR only contains one change (considered splitting up PR)
• unit-test added
• documentation updated
• CHANGELOG.md updated (only for user relevant changes)
• author name in AUTHORS

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 97.54%. Comparing base (560f84d) to head (3dc00f6).
Report is 2 commits behind head on master.

❗ Current head 3dc00f6 differs from pull request most recent head 971e6ba. Consider uploading reports for the commit 971e6ba to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1191      +/-   ##
==========================================
- Coverage   97.56%   97.54%   -0.02%     
==========================================
  Files          32       32              
  Lines        2132     2120      -12     
==========================================
- Hits         2080     2068      -12     
  Misses         52       52              

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[Andrew-Chen-Wang]

thanks!