Fix tox diffcover

[loqs]

What do these changes do?

Fix tox diffcover failure by:

• set cov-report output location to the location expected by diffcover.
• use --compare-branch master for diff-cover to match the project's branch.

Are there changes in behavior for the user?

No.

Related issue number

Related #544.

Checklist

• I think the code is well written
• Unit tests for the changes exist
• tox testenvs have been executed in the following environments:
  • Linux (Ubuntu 18.04, Ubuntu 20.04, Arch): {py36,py37,py38,py39}-diffcov
  • Windows (7, 10): {py36,py37,py38,py39}-{nocov,cov,diffcov}
  • WSL 1.0 (Ubuntu 18.04): {py36,py37,py38,py39}-{nocov,cov,diffcov}, pypy3-{nocov,cov}, qa, docs
  • FreeBSD (12.2, 12.1, 11.4): {py36,pypy3}-{nocov,cov,diffcov}, qa
  • Cygwin: py36-{nocov,cov,diffcov}, qa, docs
• Documentation reflects the changes
• Add a news fragment into the NEWS.rst file

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.84%. Comparing base (c8c4cd1) to head (0b7f43e).

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #569      +/-   ##
==========================================
- Coverage   97.89%   97.84%   -0.06%     
==========================================
  Files          23       23              
  Lines        5701     5701              
  Branches      766      766              
==========================================
- Hits         5581     5578       -3     
- Misses         75       79       +4     
+ Partials       45       44       -1     

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[webknjaz]

There's 5 failed tests in the Windows job: https://github.com/aio-libs/aiosmtpd/actions/runs/25456153086/job/74686050166#step:5:589

This seems unrelated so I've restarted the CI. Though, it's probably worth investing and digging through the logs. If the tests are flaky, they should be stabilized.

[webknjaz]

Note: I realized just now that the CI is not calling tox but invokes pytest and coverage directly, bypassing some shared configuration. I think it'd be good to start integrating reusable-tox.yml in a follow-up.

[loqs]

Though, it's probably worth investing and digging through the logs. If the tests are flaky, they should be stabilized.

I looked through the log. The failing tests are appear to Windows no allowing the same port to be reused in quick succession while this is not triggered on the Linux. I do not have a Windows system to debug the issue. I did notice some tests are already skipped on Windows.

[loqs]

Note: I realized just now that the CI is not calling tox but invokes pytest and coverage directly, bypassing some shared configuration. I think it'd be good to start integrating reusable-tox.yml in a follow-up.

I was planning on getting all current tox environments passing first. I estimate about five more pull requests four singles and a multiple commit one covering flake8 and hopefully they should all be passing.