[enh] add pyproject.toml listing build requirements

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.

pyproject.toml

@@ -0,0 +1,2 @@
+[build-system]
+requires = ["setuptools", "pyyaml"]