CI: Test on Python 3.10 and 3.11, run once a month

[EwoutH]

Some tweaks in the CI configuration:

• Add test jobs for Python 3.10 and 3.11, remove Python 3.7.
• Run once a month. This helps to detect and trace back when and why CI failed if dependencies are updated or the build environment changes.

[codecov[bot]]

Codecov Report

Merging #152 (0e3e7ac) into master (f9d62c1) will not change coverage. The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #152   +/-   ##
=======================================
  Coverage   93.33%   93.33%           
=======================================
  Files          12       12           
  Lines        1470     1470           
=======================================
  Hits         1372     1372           
  Misses         98       98           

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