Configure coveragepy to separate importables&pths

[webknjaz]

Fixes #3314.

[webknjaz]

Looks fine to me: https://app.codecov.io/github/python-trio/trio/commit/ffe7261e66a3f7af53e9671c9fc06e2342d3bda6/tree/src/trio?dropdown=coverage

[A5rocks]

Makes sense to me! Hopefully this works.

[A5rocks]

Huh, why this? I guess this is necessary to merge the importable with the file under tests/?

[webknjaz]

Well, I took wild a guess after you said that it's "installed" separately that this might've been something that you thought wasn't working in the past. I realized it's not installed but imported through an absolute path due to PYTHONPATH being set. And so I figured it would likely confuse the path mapper so I added an explicit one.
I haven't actually tested it locally even, just wanted to see what the PR would produce. Maybe it'd work even w/o this.

[webknjaz]

FWIW, looking at one of the codecov uploads (https://storage.googleapis.com/codecov-production/shelter/github/python-trio%3A%3A%3A%3Atrio/ffe7261e66a3f7af53e9671c9fc06e2342d3bda6/de0c6baf-a91a-4026-bc82-3382eb60fda8.txt?X-Amz-Algorithm=AWS4-HMAC-SHA256&X-Amz-Credential=GOOG1EJOGFN2JQ4OCTGA2MU5AEIT7OT5Z7HTFOAN2SPG4NWSN2UJYOY5U6LZQ%2F20251206%2Fus-west2%2Fs3%2Faws4_request&X-Amz-Date=20251206T014925Z&X-Amz-Expires=30&X-Amz-SignedHeaders=host&X-Amz-Signature=4623e30cb5fbffcc5f205f54c40f2a8bb795d3b626d339e6763848ec2038497f), it contains <class name="_trio_check_attrs_aliases.py" filename="tests/_trio_check_attrs_aliases.py" complexity="0" line-rate="1" branch-rate="1"> so Codecov doesn't have a problem mapping it back to the repo.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 100.00000%. Comparing base (d64798f) to head (ffe7261).
⚠️ Report is 3 commits behind head on main.

Additional details and impacted files

@@                 Coverage Diff                  @@
##                main        #3367         +/-   ##
====================================================
+ Coverage   99.98454%   100.00000%   +0.01546%     
====================================================
  Files            128          128                 
  Lines          19407        19407                 
  Branches        1318         1318                 
====================================================
+ Hits           19404        19407          +3     
+ Misses             1            0          -1     
+ Partials           2            0          -2     

see 1 file with indirect coverage changes

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