Fix issue with coverage job in GitHub Action

[tcompa]

Right now, the coverage step of ci_poetry.yml uses version of coverage different from the one used in the test_tasks version, since the former comes from a pip install and the latter is the poetry-locked one. I guess this is the reason for the failures observed by @lorenzocerrone in #866.

Meanwhile, we should also bump the version to something a bit more recent (from 6.5 to 7.6).

[tcompa]

Closed with #882

[tcompa]

Note that the actual issue is still present in #866, and looks very similar to

• Issue with Coverage Reporting: 'relative_files = true' Not Resolved py-cov-action/python-coverage-comment-action#499

[tcompa]

We provisionally fixed this with 4c12f2b, but it's clearly suboptimal as it requires installing the whole package (pytorch included) just to produce the coverage.

[tcompa]

Ref nedbat/coveragepy#876