The pyproject.toml spec is ten years old. Python 3 is almost twenty years old.
To my knowledge, for TOML spec, strict validating against a schema is not a thing. So it's not used with very likely dangerous user input.
So the spec could be 100,000 years old, for obvious security vulnerability reasons, would still not touch it.
Recommend taking a read through the strictyaml authors documentation on shortcomings of various user supplied input file formats.
You are in for a treat:
specifically Why not TOML