To run the setup.py pyyaml needs to already be installed. PEP 518
'Specifying Minimum Build System Requirements for Python Projects'
introduced the pyproject.toml file to specify the required dependencies
in a format that build tooling like pip can parse prior to executing the
setup.py.
This commit adds a pyproject.toml with the basic requirements of
setuptools and pyyaml required to run the current setup.py.
