Prefer `pyproject.toml` over `requirements.txt`

[unflxw]

It seems that most Python projects have migrated to using pyproject.toml. If a pyproject.toml file exists, our installer should probably write the dependencies to that file, instead of to requirements.txt.

(In the absence of either of the files, creating a requirements.txt is fine, as creating a pyproject.toml is a whole thing)

• Installer supports pyproject.toml and uses it when it exists
• Docs mention pyproject.toml alongside requirements.txt for manual installation