Add requires-python to uv init

[charliermarsh]

Summary

Prefers, in order:

• The major-minor version of an interpreter discovered via --python.
• The requires-python from the workspace.
• The major-minor version of the default interpreter.

If the --python request is a version or a version range, we use that without fetching an interpreter.

Closes #5299.

[charliermarsh]

@zanieb - Can you review that what I'm doing with the discovery here is reasonable?

[FishAlchemist]

Should we avoid creating pyproject.toml when providing python version causes an error?
This is because creating it would lead to Project is already initialized.

A failed creation will result in pyproject.toml being created, but missing requires-python
e.g.

[project]
name = "www"
version = "0.1.0"

[charliermarsh]

Yeah... Though unfortunately we need to create the pyproject.toml to discover the workspace :(

I'll figure something out.

[j178]

Just a heads-up,cargo init checks workspace.members glob pattern before creating the project:

https://github.com/rust-lang/cargo/blob/7f2079838ac6e0a3cb05b1b9dac4401a76572855/src/cargo/ops/cargo_new.rs#L978-L996

[charliermarsh]

@j178 - We also check that before modifying the workspace manifest.