Use Python 3.9 in all CI/CD workflows

[CasperWA]

Description

Use Python 3.9 throughout all CI/CD workflows.

Fixes #193

Type of change

• Bug fix and code cleanup
• New feature
• Documentation update
• Testing

Checklist for the reviewer

This checklist should be used as a help for the reviewer.

• Is the change limited to one issue?
• Does this PR close the issue?
• Is the code easy to read and understand?
• Do all new feature have an accompanying new test?
• Has the documentation been updated as necessary?
• Is the code properly tested?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.47%. Comparing base (f04c50f) to head (54bfab5).

Additional details and impacted files

@@           Coverage Diff           @@
##           master     #196   +/-   ##
=======================================
  Coverage   77.47%   77.47%           
=======================================
  Files          18       18           
  Lines        1545     1545           
=======================================
  Hits         1197     1197           
  Misses        348      348           

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[ajeklund]

Thank you!