Fix for resolving source when accessing sub-attr eg b.c

[cosmosgenius]

plan = PlanSerializer(source='subscription.plan')

plan is a field of another serializer which needs to be resolved using obj.subscription.plan property. It works for standard DRF JSON response but fails in drf-json-api.

This PR fixes that issue by replacing getattr with operator.attrgetter which is able to resolve the sub-attr access.

[cosmosgenius]

ping!. Is there anything else which needs to be done to merge this PR?

[mblayman]

Hi, @cosmosgenius, thanks for your patience. Anyone working on this project is purely a volunteer so I guess we've all been fairly busy lately. Through no fault of your own, the tests are all failing. I believe this is because of a dependency problem that was resolved recently. Could you merge the latest develop into this branch? The project policy is to merge PRs only when they pass all tests. Once this branch is passing, I'm happy to merge as the fix content looks good. Thanks! 👍

[codecov-io]

Codecov Report

Merging #375 into develop will increase coverage by <.01%.
The diff coverage is 100%.

@@             Coverage Diff             @@
##           develop     #375      +/-   ##
===========================================
+ Coverage    93.05%   93.06%   +<.01%     
===========================================
  Files           51       51              
  Lines         2665     2666       +1     
===========================================
+ Hits          2480     2481       +1     
  Misses         185      185

Impacted Files	Coverage Δ
rest_framework_json_api/utils.py	90.8% <100%> (+0.03%)	⬆️

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[cosmosgenius]

@mblayman no issues. Just checking up :). I have rebased the branch with the latest develop and fixed an isort issue. Let me know if anything else is required.

[mblayman]

Awesome. Thanks, @cosmosgenius!