add pyproject.toml for numpy requirement

[valrus]

This PR adds a pyproject.toml file to address the need to manually install numpy before aeneas, which was preventing aeneas from installing correctly in a Heroku app.

I took the approach from this PR on a project with the same issue. Based on the comments on that PR, it should work for any version of pip above 10 and be harmless for lower versions. I tested it in a Heroku deploy and was able to install aeneas directly from a requirements.txt file.

[EthanShoeDev]

A recent update to setuptools broke this fix for me. I created a fork that fixes the issue by pinning setuptools to 75.8.x