Add Python 3.11 to supported versions, add to tests

[danielhuppmann]

Description of PR

In response to #744, this PR adds Python 3.11 to supported versions and the tests.

[codecov[bot]]

Codecov Report

Merging #745 (fa5964b) into main (d6d06db) will increase coverage by 0.0%. The diff coverage is n/a.

@@          Coverage Diff          @@
##            main    #745   +/-   ##
=====================================
  Coverage   95.1%   95.1%           
=====================================
  Files         59      59           
  Lines       6006    6017   +11     
=====================================
+ Hits        5713    5724   +11     
  Misses       293     293           

see 2 files with indirect coverage changes