use field_name for DeferredAttributes to be version agnostic

[v-erena]

As Django docs suggest, you can just use the field name of a model field to return the field instance.
https://docs.djangoproject.com/en/3.0/ref/models/meta/#django.db.models.options.Options.get_field

This makes the attribute doc retrieval for a DeferredAttribute easier as it is Django version agnostic.

Note: not quite sure whether I instantiated the DeferredAttribute correctly in the new test. Please have an eye on that.

[codecov-io]

Codecov Report

Merging #22 into master will increase coverage by 5.39%.
The diff coverage is 85.71%.

@@            Coverage Diff             @@
##           master      #22      +/-   ##
==========================================
+ Coverage   57.28%   62.67%   +5.39%     
==========================================
  Files           6        6              
  Lines         206      217      +11     
==========================================
+ Hits          118      136      +18     
+ Misses         88       81       -7

Impacted Files	Coverage Δ
sphinxcontrib_django/docstrings.py	46.97% <100%> (+5.25%)	⬆️
sphinxcontrib_django/tests/test_docstrings.py	96.15% <84.61%> (-3.85%)	⬇️

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[vdboor]

Thanks a lot!

I've released this as v0.5.1