Fix code coverage

[flying-sheep]

PR Checklist

• Fixes codecov not working anymore #428
• No tests necessary
• No doc changes necessary

Description of changes

• Changing things so CI fails when coverage collection failed (0%) or upload failed
• Switch to run coverage directly instead of using pytest-cov

Technical details

Using pytest-cov can sometimes cause problems since it initializes its machinery after pytest has started doing its thing. coverage run will work in those circumstances.

Of course that means that pytest can’t prevent adding the current working directory to PYTHONPATH unless the project uses the src layout. Experimentally switching the project to src layout worked, but I think that’s out of scope for this PR.

Additional context

[codecov]

Codecov Report

Merging #432 (bd30b11) into main (9f96549) will increase coverage by 58.23%.
The diff coverage is n/a.

❗ Current head bd30b11 differs from pull request most recent head 5943625. Consider uploading reports for the commit 5943625 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##            main     #432       +/-   ##
==========================================
+ Coverage   0.00%   58.23%   +58.23%     
==========================================
  Files         40       40               
  Lines       4899     4899               
==========================================
+ Hits           0     2853     +2853     
+ Misses      4899     2046     -2853     

see 39 files with indirect coverage changes

[Zethson]

Thank you very much!