Prevent deepcopying of filter's parent

[rpkilby]

It's unnecessary and costly to deepcopy the filter's parent and model. This should fix #171 as a side effect.

[codecov-io]

Codecov Report

Merging #172 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #172      +/-   ##
==========================================
+ Coverage   99.53%   99.53%   +<.01%     
==========================================
  Files           6        6              
  Lines         215      217       +2     
  Branches       42       42              
==========================================
+ Hits          214      216       +2     
  Partials        1        1

Impacted Files	Coverage Δ
rest_framework_filters/filterset.py	99.14% <100%> (+0.01%)	⬆️

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