Fix poetry.lock: poetry lock --no-update

The pyproject.toml and the poetry.lock files were out of sync, so to
resolve them, I did a `poetry lock --no-update`.

This added the `yaffshiv` package to the lock file, which was missing and
causing the out-of-sync behavior. For some magic reason, it also updated
the `coverage` from 6.2 to 6.3, resulting the minimum python package
increase from 3.6 to 3.7.

python-poetry/poetry#1614