Add python 3.11 and drop python 3.6 support

[Pliner]

What do these changes do?

Are there changes in behavior for the user?

Related issue number

Checklist

• I think the code is well written
• Unit tests for the changes exist
• Documentation reflects the changes
• Add a new news fragment into the CHANGES folder
  • name it <issue_id>.<type> (e.g. 588.bugfix)
  • if you don't have an issue_id change it to the pr id after creating the PR
  • ensure type is one of the following:
    • .feature: Signifying a new feature.
    • .bugfix: Signifying a bug fix.
    • .doc: Signifying a documentation improvement.
    • .removal: Signifying a deprecation or removal of public API.
    • .misc: A ticket has been closed, but it is not of interest to users.
  • Make sure to use full sentences with correct case and punctuation, for example: Fix issue with non-ascii contents in doctest text files.

[codecov]

Codecov Report

Merging #892 (6b90d49) into master (1ba66ca) will not change coverage.
The diff coverage is n/a.

❗ Current head 6b90d49 differs from pull request most recent head 44758b1. Consider uploading reports for the commit 44758b1 to get more accurate results

@@           Coverage Diff           @@
##           master     #892   +/-   ##
=======================================
  Coverage   91.77%   91.77%           
=======================================
  Files           9        9           
  Lines         863      863           
  Branches      118      118           
=======================================
  Hits          792      792           
  Misses         53       53           
  Partials       18       18           

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