Add missing comma in setup.py

[emlaver]

PR summary

Upload to pypi fails during release because of missing comma:

2021-02-10 14:30:52.934 |  HTTPError: 400 Bad Request from https://upload.pypi.org/legacy/
2021-02-10 14:30:52.935 |  Invalid value for classifiers. Error: Classifier 'Programming Language :: Python :: 3.9Development Status :: 4 - Beta' is not a valid classifier.

PR Checklist

Please make sure that your PR fulfills the following requirements:

• The commit message follows the
  Angular Commit Message Guidelines.
• Tests for the changes have been added (for bug fixes / features)
• Docs have been added / updated (for bug fixes / features)

PR Type

• Bugfix
• Feature
• Code style update (formatting, local variables)
• Refactoring (no functional changes, no api changes)
• New tests
• Build/CI related changes
• Documentation content changes
• Other (please describe)

What is the current behavior?

What is the new behavior?

Does this PR introduce a breaking change?

• Yes
• No

Other information

[codecov]

Codecov Report

Merging #76 (11c86d8) into master (ad34ab4) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #76   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files           8        8           
  Lines        7085     7085           
=======================================
  Hits         6539     6539           
  Misses        546      546           

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