* gnu/packages/python-xyz.scm (python-apispec): Update to 6.0.2.
[build-system]: Use pyproject-build-system.
[arguments]: Disable test through test-flags, not by replacing the 'check
phase.
[propagated-inputs]: Remove python-pyyaml; add python-packaging.
[native-inputs]: Add python-flake8, python-flake8-bugbear, python-mypy,
python-pre-commit, and python-pyyaml.