WIP: tests: Add cron for running toxgen

[sentrivana]

Run toxgen weekly and have it submit a PR.

TODO:

• test that the action works and has the right privileges
• only open the PR if something has actually changed (i.e., not just the generated timestamp)

Closes #4050

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 79.53%. Comparing base (189e4a9) to head (aa49ee9).

✅ All tests successful. No failed tests found.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4071      +/-   ##
==========================================
- Coverage   79.55%   79.53%   -0.02%     
==========================================
  Files         140      140              
  Lines       15521    15521              
  Branches     2631     2631              
==========================================
- Hits        12347    12344       -3     
- Misses       2338     2340       +2     
- Partials      836      837       +1     

see 2 files with indirect coverage changes