drop six as dependency

[mathause]

Please confirm that this PR has done the following:

• Tests Added
• Documentation Added
• Name of contributors Added to AUTHORS.rst
• Description in RELEASE_NOTES.md Added

Adding to RELEASE_NOTES.md (remove section after adding to RELEASE_NOTES.md)

Please add a single line in the release notes similar to the following:

- (#XX)[http://link-to-pr.com] Added feature which does something

Description of PR

As you require python 3+ you can drop the dependency on six (I think the poetry.lock file does not need any updates, but let me know if it does). I can add AUTHORS.rst and RELEASE_NOTES.md if you want to go ahead with this change.

[mathause]

The changes to poetry.lock are unrelated but seems necessary to update it anyways.

[mathause]

Test failure are unrelated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.0%. Comparing base (ddbb88e) to head (6ab9404).
Report is 12 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##            main    #846     +/-   ##
=======================================
- Coverage   95.0%   95.0%   -0.1%     
=======================================
  Files         64      63      -1     
  Lines       6134    6148     +14     
=======================================
+ Hits        5828    5841     +13     
- Misses       306     307      +1     

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[danielhuppmann]

Thanks @mathause!