setuptools now requires a pyproject.toml file to specify dependencies

[edaub]

setuptools (which we use in the setup.py file to configure the build) needs to have any dependencies for the build specified somewhere. This is done through a pyproject.toml file. The GPU build requires one additional dependency (numpy to get the include path for pybind11), so the build fails without a pyproject.toml file when building a distribution to upload to PyPI.

I have created a temporary local file on my computer that fixes this problem for the sake of uploading to PyPI. I have checked that the uploaded version installs correctly into a fresh virtual environment, but I want to be sure I understand what should be specified in the pyproject.toml file before committing one to the repo. Will be sure to sort this for merging version 0.7!