devel/py-pytest-cov: Update to 6.3.0

Adjust the dependencies to what upstream now requires and drop the six
test dependency, which is no longer used. The sdist was renamed to use
an underscore, so set DISTNAME accordingly.

Newer versions require devel/py-coverage 7.10.6 or later, which is not
in the tree yet.

Changes: https://github.com/pytest-dev/pytest-cov/blob/v6.3.0/CHANGELOG.rst