Simple clean up changes.

[sdolemelipone]

1. Moved BaseFormSetMixin.success_url to FormSetMixin.success_url, as this specifically relates to functionality of a subclassed view.
2. Removed ModelFormSetMixin.get_context_data() method as django now uses self.object_list correctly (rather than requiring it to be in **kwargs which was previously the case).
3. ModelFormSetMixin.get_factory_kwargs and InlineFormSetMixin.get_factory_kwargs were both calling self().get_formset_class() a second time even though this would already have been called by BaseFormSetMixin.factory_kwargs() as part of the super() call. The extra calls have been removed.
4. Removed explicit setting of BaseInlineFormSetMixin.formset_class and GenericInlineFormSetMixin.formset_class as the defaults for inlineformset_factory and genericinlineformset_factory set this automatically.
5. Removed BaseInlineFormSetMixin.get_context_data as this will already be set by SingleObjectMixin.get_context_data when subclassed.
6. Switched subclassing of InlineFormSetMixin so that SingleObjectMixin.get_context_data will be called ahead of FormSetMixin.get_context_data (as ContextMixin.get_context_data does not super() itself).

[sdolemelipone]

Partly raised due to #165.

[codecov-io]

Codecov Report

Merging #166 into master will decrease coverage by 0.55%.
The diff coverage is 33.33%.

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
- Coverage   53.34%   52.79%   -0.56%     
==========================================
  Files           6        6              
  Lines         508      483      -25     
  Branches       62       56       -6     
==========================================
- Hits          271      255      -16     
+ Misses        216      212       -4     
+ Partials       21       16       -5

Impacted Files	Coverage Δ
extra_views/formsets.py	46.56% <0%> (-2.47%)	⬇️
extra_views/generic.py	100% <100%> (ø)	⬆️

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