Add python 3.8 support

[Djailla]

No description provided.

[codecov-io]

Codecov Report

Merging #1138 into master will increase coverage by 0.03%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1138      +/-   ##
==========================================
+ Coverage    98.1%   98.13%   +0.03%     
==========================================
  Files          15       15              
  Lines        1215     1235      +20     
==========================================
+ Hits         1192     1212      +20     
  Misses         23       23

Impacted Files	Coverage Δ
django_filters/filterset.py	100% <0%> (ø)	⬆️
django_filters/rest_framework/filterset.py	100% <0%> (ø)	⬆️
django_filters/filters.py	99.71% <0%> (ø)	⬆️
django_filters/rest_framework/backends.py	91.56% <0%> (+0.1%)	⬆️

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[carltongibson]

Great thanks. Welcome aboard! 🚀