`yamllint/conf/*.yml` remains in `setup.py`'s `package_data` because it
needs to be installed when running `pip install .`.
`docs/*` and `tests/yaml-1.2-spec-examples/*` just need to be packaged,
they can go in the manifest.
The `pyyaml` dependency is needed in `install_requires` but also in
`setup_requires`, because running `setup.py` requires importing
`yamllint`, which itself imports `yaml`.
