add py39 and setup_py to setup_cfg

[jkittner]

Hi,
thanks for your great tool! I added the new python 3.9 to the classifiers and to the ci. I also converted the old setup.py to the declarative setup.cfg format containing all information.

[codecov-io]

Codecov Report

Merging #57 (47c0af8) into primary (df5dbf9) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff            @@
##           primary      #57   +/-   ##
========================================
  Coverage    83.77%   83.77%           
========================================
  Files            1        1           
  Lines          228      228           
========================================
  Hits           191      191           
  Misses          37       37           

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[JoshData]

Neat!