[TODO] Fix coverage collection for Cython modules

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### Describe the bug

Some time between https://app.codecov.io/gh/aio-libs/yarl/commit/d1286dbe71ea1caaf7c8e488b608ae1637702e90 (https://github.com/aio-libs/yarl/commit/d1286dbe71ea1caaf7c8e488b608ae1637702e90) and https://app.codecov.io/gh/aio-libs/yarl/commit/63c446174218bdcd31274b4488e513de1a05c54e (https://github.com/aio-libs/yarl/commit/63c446174218bdcd31274b4488e513de1a05c54e) Codecov forgot about the existance of Cython modules entirely.
Need to bisect that and see what happened.

### To Reproduce

🤷‍♂️

### Expected behavior

`*.pyx` files should show coverage.

### Additional context

The same thing is happening in another project with identical setup: https://github.com/ansible/pylibssh/issues/781.

The only thing suspicious in the diff is the changes to the `linetrace` setting: https://github.com/aio-libs/yarl/compare/d1286db...63c4461. However, `ansible-pylibssh` still has this setting on and has this problem nevertheless. So I think this might be unreleated.

My guess is that perhaps Cython was unpinned at the time and we didn't notice when this changed.