Deprecate 'extra' field attrs on Filter

[rpkilby]

I was working on some related code and it occurred to me that filters don't generally make use of the required and widget attributes. It doesn't necessarily seem beneficial to store them separately from the extra form field kwargs.

[codecov-io]

Codecov Report

Merging #734 into develop will increase coverage by 0.1%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           develop     #734     +/-   ##
==========================================
+ Coverage    98.11%   98.22%   +0.1%     
==========================================
  Files           15       15             
  Lines         1117     1127     +10     
==========================================
+ Hits          1096     1107     +11     
+ Misses          21       20      -1

Impacted Files	Coverage Δ
django_filters/filters.py	98.52% <100%> (+0.04%)	⬆️
django_filters/utils.py	99.1% <0%> (+0.89%)	⬆️

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[rpkilby]

rebased

[carltongibson]

Yes. This makes sense. Thanks!