implement DjangoFilterBackend.get_schema_operation_parameters

[n2ygk]

Fixes #644

Description of the Change

Implements filter backend functionality required by DRF 3.9+ generateschema management command. Eventually this can extend django-filter's implementation, if and when carltongibson/django-filter#1086 is merged, but it's small enough to stand on its own.

See #604 for more stuff still needed but it shouldn't hurt to merge this now. It will at least prevent an exception from being raised if someone tries to generateschema although the output will not be correct until the rest of #604 is done.

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

Merging #646 into master will decrease coverage by 0.34%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
- Coverage   96.15%   95.81%   -0.35%     
==========================================
  Files          57       58       +1     
  Lines        2889     2913      +24     
==========================================
+ Hits         2778     2791      +13     
- Misses        111      122      +11

Impacted Files	Coverage Δ
example/tests/unit/test_openapi.py	100% <100%> (ø)
rest_framework_json_api/django_filters/backends.py	83.33% <66.66%> (-16.67%)	⬇️
rest_framework_json_api/compat.py	50% <0%> (-50%)	⬇️

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[codecov]

Codecov Report

Merging #646 into master will decrease coverage by 0.22%.
The diff coverage is 22.22%.

@@            Coverage Diff             @@
##           master     #646      +/-   ##
==========================================
- Coverage   96.15%   95.93%   -0.23%     
==========================================
  Files          57       56       -1     
  Lines        2889     2879      -10     
==========================================
- Hits         2778     2762      -16     
- Misses        111      117       +6

Impacted Files	Coverage Δ
rest_framework_json_api/django_filters/backends.py	87.03% <22.22%> (-12.97%)	⬇️
example/tests/integration/test_polymorphism.py	100% <0%> (ø)	⬆️
example/models.py	100% <0%> (ø)	⬆️
example/tests/unit/test_renderers.py	100% <0%> (ø)	⬆️
example/migrations/0007_artproject_description.py
rest_framework_json_api/renderers.py	87.37% <0%> (+0.57%)	⬆️

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[n2ygk]

I'm going to cancel this PR as I'm close to submitting a more complete openapi generateschema PR and will incorporate this related code there.