Add py39 to github workflows

[katxiao]

issue #631

[amontanez24]

While you're doing this, can we add 3.9 here as well?

SDV/setup.py

Lines 89 to 93 in 08bed6d

	'Programming Language :: Python :: 3',
	'Programming Language :: Python :: 3.6',
	'Programming Language :: Python :: 3.7',
	'Programming Language :: Python :: 3.8',
	],

I missed this in the original PR

[codecov-commenter]

Codecov Report

Merging #649 (76324e7) into master (08bed6d) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #649   +/-   ##
=======================================
  Coverage   64.92%   64.92%           
=======================================
  Files          34       34           
  Lines        2583     2583           
=======================================
  Hits         1677     1677           
  Misses        906      906           

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[amontanez24]

LGTM! Thanks for finding this and fixing!