Add pyproject.toml

[nritsche]

pyproject.toml should list python packages needed for setup:

[build-system]
requires = ['setuptools', 'wheel', 'cython', 'numpy', 're', 'distutils', ...]

Once this is done, .travis.yaml in e.g. https://github.com/radiocosmology/draco doesn't have to install those manually.

[ljgray]

This was added