Upper bound for requires-python

[timothyngo]

The requires-python field in pyproject.toml restricts the version to be less than 4. This is necessary, because Poetry and many other package managers have a very strict solver and removing this line will cause some packages to get downgraded unnecessarily. See here for a discussion of the behavior and interpretation of the requires-python field.

Ideally we eventually remove this <4 condition when we have a build and lock system that supports it, but this may require moving to another package manager like uv.