Update supported Python versions

[emlaver]

PR summary

Drop support for Python 3.5, add Python 3.9

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)

Does this PR introduce a breaking change?

• Yes
• No

Other information

[codecov]

Codecov Report

Merging #75 (0e07e1b) into master (00fb735) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master      #75   +/-   ##
=======================================
  Coverage   92.29%   92.29%           
=======================================
  Files           8        8           
  Lines        7085     7085           
=======================================
  Hits         6539     6539           
  Misses        546      546           

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[bessbd]

Related PRs: #70 , #69 and #76 just so that people find the reference when they want to bump again in the future.