Add trove classifier for Python 3.8 support

[jdufresne]

Python 3.8 was released October 14th, 2019.

[codecov]

Codecov Report

Merging #1290 into master will increase coverage by 0.13%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #1290      +/-   ##
==========================================
+ Coverage   86.61%   86.74%   +0.13%     
==========================================
  Files          25       25              
  Lines        1449     1449              
  Branches      205      205              
==========================================
+ Hits         1255     1257       +2     
+ Misses        142      141       -1     
+ Partials       52       51       -1     

Impacted Files	Coverage Δ
debug_toolbar/panels/profiling.py	91.07% <0.00%> (+1.78%)	⬆️

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