Schedule CI to run automatically every Sunday

[omus]

I noticed that the CI last ran over 2 years ago. This is so old that the CI badge in the README no longer shows passing as all of the workflow logs have expired. This PR makes the CI run at least once a week.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (f1eaaf9) 95.65% compared to head (3005895) 95.67%.

❗ Current head 3005895 differs from pull request most recent head 22ac2ae. Consider uploading reports for the commit 22ac2ae to get more accurate results

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Additional details and impacted files

@@            Coverage Diff             @@
##           master     #125      +/-   ##
==========================================
+ Coverage   95.65%   95.67%   +0.02%     
==========================================
  Files           5        5              
  Lines        1381     1296      -85     
==========================================
- Hits         1321     1240      -81     
+ Misses         60       56       -4     

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[carlobaldassi]

Thanks!