[red-knot] Fallback to requires-python if no python-version is specified

[MichaReiser]

Summary

Add support for the project.requires-python field in pyproject.toml files.

Fall back to the resolved lower bound of project.requires-python if the environment.python-version field is None (or more accurately, initialize environment.python-version with requires-python`'s lower bound if left unspecified).

UX design

There are two options on how we can handle the fallback to requires-python's lower bound:

1. Store the resolved lower bound in environment.python-version if that field is None (Implemented in this PR)
2. Store the requires-python constraint separately.

There's no observed difference unless a user-level configuration (or any other inherited configuration is used). Let's discuss it on the given example

User configuration

[environment]
python-version = "3.10"

Project configuration (pyproject.toml)

[project]
name = "test"
requires-python = ">= 3.12"

[tool.knot]
# No environment table

The resolved version for 1. is 3.12 because the requires-python constraint precedence takes precedence over the python-version in the user configuration. 2. resolves to 3.10 because all python-version constraints take precedence before falling back to requires-python.

Ruff implements 1. It's also the easier to implement and it does seem intuitive to me that the more local requires-python constraint takes precedence.

Test plan

Added CLI and unit tests.

[MichaReiser]

Thanks to @konstin for pointing me toward the logic in uv. This matches what uv does. Thankfully, most of the work is done by the pep0440 crate.

[sharkdp]

Thank you. A few minor comments and one higher-level question.

[AlexWaygood]

Nice!

[AlexWaygood]

They might not have a tool.knot table at all, right? This could be quite confusing if they aren't familiar with how to provide this red-knot-specific setting

Suggested change

	#[error("value `{0}` does not contain a lower bound. Add a lower bound to indicate the minimum compatible Python version (e.g., `>=3.13`) or specify a version in `environment.python-version`.")]
	#[error("value `{0}` does not contain a lower bound. Add a lower bound to indicate the minimum compatible Python version (e.g., `>=3.13`) or specify a version in `tool.knot.environment.python-version`.")]