Set codecov-action back to v3

[simleo]

After merging #59, the CI workflow failed with:

Error: Unable to resolve action `codecov/codecov-action@v4`, unable to find version `v4`

[codecov]

Codecov Report

Patch and project coverage have no change.

Comparison is base (d00a6bb) 97.61% compared to head (af817b0) 97.61%.
Report is 2 commits behind head on main.

Additional details and impacted files

@@           Coverage Diff           @@
##             main      #60   +/-   ##
=======================================
  Coverage   97.61%   97.61%           
=======================================
  Files          12       12           
  Lines        1930     1930           
=======================================
  Hits         1884     1884           
  Misses         46       46           

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