Use the admin obj's hook methods (get_fieldsets, etc) instead of the MassAdmin's own

[pedrovhb]

Hello! First off, thanks for the library, it's quite useful :slightly_smiling_face:

This PR aims to change forms so that they use the admin_obj's get_fieldsets, get_prepopulated_fields, and get_readonly_fields.

Currently, behavior is inconsistent with regards to these fields (I'll be using fieldsets as an example but the same applies to both other methods). As a refresher, defining fieldsets in a model admin makes the page use the provided fieldsets; implementing get_fieldsets uses the return value from that, instead. The non-overridden behavior of get_fieldsets is to just return fieldsets.

In massadmin, non-function attributes are copied over in get_overrided_properties, so defining fieldsets will use the copied attribute and work as expected. However, since the method called is MassAdmin.get_fieldsets, we always get the non-overridden behavior of just using fieldsets, and the model admin's get_fieldsets is ignored.

If we use self.admin_obj.get_fieldsets instead of self.get_fieldsets, then we get the expected overriden call if it is defined by the model admin class, and the default (current) behavior if it is not.

Thanks!

cc @PetrDlouhy

[codecov-commenter]

Codecov Report

Merging #103 (c8bdcaf) into master (3787308) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #103   +/-   ##
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  Coverage   88.37%   88.37%           
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  Files           6        6           
  Lines         215      215           
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  Hits          190      190           
  Misses         25       25           

Impacted Files	Coverage Δ
massadmin/massadmin.py	86.63% <ø> (ø)

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[PetrDlouhy]

@pedrovhb Looks very nice. Thank you very much.

[pedrovhb]

@PetrDlouhy happy to contribute!

@bufke I'm not familiar with how release cycles work for the library, is there anything I can expect in terms of a timeline for a public PyPI release? No pressure at all, just to know what to plan for :)