Refactor backend filterset instantiation

[rpkilby]

Possible compromise for #857. Right now, overriding the filterset is actually pretty inconvenient, because there is no hook into its instantiation aside from getting the class. To override the instance, it is necessary to completely overwrite both filter_queryset and to_html.

[rpkilby]

Side note - @carltongibson I've noticed for a while that the CI builds seem to run jobs one at a time, instead of the usual five concurrently. Have current jobs been disabled/limited in the settings?

[codecov-io]

Codecov Report

Merging #865 into master will decrease coverage by 0.54%.
The diff coverage is 89.47%.

@@            Coverage Diff             @@
##           master     #865      +/-   ##
==========================================
- Coverage   98.41%   97.87%   -0.55%     
==========================================
  Files          15       15              
  Lines        1198     1128      -70     
==========================================
- Hits         1179     1104      -75     
- Misses         19       24       +5

Impacted Files	Coverage Δ
django_filters/rest_framework/backends.py	93.75% <89.47%> (-1.84%)	⬇️
django_filters/utils.py	99.02% <0%> (-0.98%)	⬇️
django_filters/filters.py	98.42% <0%> (-0.97%)	⬇️
django_filters/fields.py	100% <0%> (ø)	⬆️
django_filters/views.py	100% <0%> (ø)	⬆️
django_filters/filterset.py	100% <0%> (ø)	⬆️
django_filters/rest_framework/filterset.py	100% <0%> (ø)	⬆️
django_filters/rest_framework/filters.py	100% <0%> (ø)	⬆️

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[carltongibson]

Yea. OK. I could see subclassing the backend quickly next to the view.

Could you add notes to the docs?

[rpkilby]

Could you add notes to the docs?

Do you mean for the new methods? get_filter_class isn't currently documented.
Or do you mean a migration note for the get_filter_class => get_filterset_class rename?

[carltongibson]

I meant for the new methods. (But yes...)

(Build should be fixed BTW. I must have been on the mezcal.)

[rpkilby]

Should View.filter_class and View.filter_fields be updated to .filterset_class and .filterset_fields?

This might be a good time to unify the DRF and Django view kwargs. The terminology is a little inconsistent, going between filter_* and filterset_*

[carltongibson]

Should ... be updated ...

It's probably a good idea yes. And 2.0 is the time for that. Can we raise a warning (to be removed in 2.1)?

[rpkilby]

Note: After #867 is merged, this need to be rebased to account for the method renames/deprecations.

[lukyoung]

Why didn't you pass the view argument into the returning data of the method django_filters.rest_framework.backends.DjangoFilterBackend.get_filterset_kwargs ?