Drop Py 3.6 support and add 3.8

[ibnesayeed]

Hopefully, it will change the metadata on PyPI and close #652.

[codecov]

Codecov Report

Merging #653 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #653   +/-   ##
=======================================
  Coverage   27.28%   27.28%           
=======================================
  Files           7        7           
  Lines        1246     1246           
  Branches      191      191           
=======================================
  Hits          340      340           
  Misses        881      881           
  Partials       25       25           

Impacted Files	Coverage Δ
setup.py	0.00% <ø> (ø)

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[machawk1]

Good catch.