Add `coverage-conditional-plugin`

[graingert]

          Consider integrating the `covdefaults` plugin, as it injects most of these automatically. The only thing you'd need to override additionally is the `fail_under` setting, which it sets to 100%, but it's not compatible with runtime-dependent tests that cannot reach 100% under one specific runtime. So it's good to set it to something sufficiently low while requiring 100% in Codecov.

Originally posted by @webknjaz in #3159 (comment)

[graingert]

https://github.com/asottile/covdefaults looks handy as it configures a bunch of nice settings

I'm not sure I agree with all the no-cov pragmas, however.

[jakkdl]

Ooh, the platform/version pragmas are a really interesting way of enforcing 100% coverage for each individual run. Maybe not for trio, but will consider it for projects which have a very small amount of platform/version-dependent code. This should be added to upstream coverage tbh

But yeah I'm not finding a way of removing some of the exclude_lines... looks like @webknjaz already opened an issue about that: asottile/covdefaults#152

[graingert]

uvicorn uses an interesting approach: https://github.com/encode/uvicorn/blob/master/pyproject.toml#L124

[jakkdl]

I also found coveragepy/coveragepy#1699 which mentioned https://pypi.org/project/coverage-simple-excludes/ and https://github.com/beeware/briefcase/blob/a898beaed7126f07b3a0ae21df8b291d512f798f/pyproject.toml#L202 (which, like uvicorn, uses https://pypi.org/project/coverage-conditional-plugin/)

[webknjaz]

Oh, nice! More projects to try out for me :)

[jakkdl]

I've added coverage-conditional-plugin to flake8-async with python-trio/flake8-async#356. It works well - but big warning that the examples in the README are incorrect wemake-services/coverage-conditional-plugin#227

[jakkdl]

Let's transition this issue to being about coverage-conditional-plugin, seeing as we don't agree with upstream covdefaults and can replicate any settings from it that we like.

[webknjaz]

FTR looks like we might be building another plugin soon: jazzband/pip-tools#2264. Not sure if it'll be relevant here but linking just in case.