Add pyproject.toml

[babsey]

Use pyproject.toml to build Python package (PEP 621).

[terhorstd]

Hi @babsey,
Thanks for updating the package structure! Check that under src/ we would want to have another level with the module name, so for example src/nest_client/__init__.py. Then most build tools find stuff and put it in the right place. Details on the python packaging and pages around there.

[babsey]

@terhorstd I am not able to add you as reviewer.

I would ask you to review it before Hackathon in December.