Address invisible hidden files in artifact upload action

[seifertm]

Coverage writes reports to ".coverage" by default. The GitHub action "upload-artifact" no longer includes hidden files by default. This patch changes the coverage file name to a non-hidden file path.

This fixes the failing CI pipeline for pull requests.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 91.10%. Comparing base (fb5422f) to head (cd06de6).
Report is 20 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #941      +/-   ##
==========================================
- Coverage   91.30%   91.10%   -0.20%     
==========================================
  Files           2        2              
  Lines         506      506              
  Branches       98       98              
==========================================
- Hits          462      461       -1     
- Misses         26       27       +1     
  Partials       18       18              

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[Tinche]

Nb: you could have also reenabled hidden file upload in the action with a flag

[seifertm]

@Tinche Thanks for the review!

Before this PR, each tox environment created its own coverage file in the project root directory. They don't get overwritten either, so the developer has to clean up the files manually every now and then. I found this a bit annoying.

I considered the include-hidden-files option for actions/upload-artifact, but I saw this as a chance to group the coverage files for each tox environment into a single folder to avoid messing up the file explorer for devs :)