Introspection: add a CLI to generate stubs

[Tpt]

Useful for setuptools-rust

Use it also for the integration tests

[davidhewitt]

Thanks, the binary implementation looks fine to me (and matches what I've done locally in testing previously).

Some thoughts / questions:

• Will we ship prebuilt versions of this, e.g. on this repository? If no, will setuptools-rust build it from scratch?
• Does this affect any planned integration with maturin (e.g. make it irrelevant?)
• Should we document how to use this in type-stub.md?

[Tpt]

Will we ship prebuilt versions of this, e.g. on this repository? If no, will setuptools-rust build it from scratch?

Good question. Building in this repository and publish on pypi is likely the easiest and nicest way to go. This way setuptools-rust can just depend on it. Glad to do the basic setup if you are ok with this idea.

Does this affect any planned integration with maturin (e.g. make it irrelevant?)

No, maturin integration is imho still very relevant as an "integrated" tool doing everything (build + introspection). We just merged PyO3/maturin#3105 with first integration bit (a --generate-stubs option that adds stubs to the built artifacts).

Should we document how to use this in type-stub.md?

Definitely. Waiting for the next release is maybe better to not confuse the reader by talking about a not released tool. WDYT?