Add Python 3.8 & 3.9 support

[eumiro]

Add py38 and py39 to tox.ini.

Would you consider dropping support for 2.7, 3.4, and 3.5 (all of them after EOL)?

[codecov]

Codecov Report

Merging #763 (d4fde4e) into master (9f6ea69) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #763   +/-   ##
=======================================
  Coverage   91.12%   91.12%           
=======================================
  Files          24       24           
  Lines        4180     4180           
=======================================
  Hits         3809     3809           
  Misses        371      371           

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[akx]

This was done in #754 (which I forgot to merge). Thanks though!