examples: maturin and setuptools_rust examples

[davidhewitt]

This builds upon my experience writing the maturin and setuptools_rust examples in #1269

This time I rearrange the example modules as follows:

pyo3_benchmarks -> pyo3-benchmarks
rustapi_module  -> pyo3-pytests
*new example*      maturin-starter
*new example*      setuptools-rust-starter

The two existing packages got a bit of a clean-up. The two new examples maturin-starter and setuptools-rust-starter are copies of each other tailored to the two different build systems.

I made good use of the trick in #1517 (comment) to make importing the submodules easy, and I wanted to show this off in all the examples. (e.g. from maturin_starter.submodule import SubmoduleClass).

Closes #807

[davidhewitt]

cc @konstin I've finally almost completed a working set of maturin examples for PyO3!

Unfortunately it appears that maturin doesn't build under MSRV. That's ok as long as there's a wheel to install from. I'll need PyO3/maturin#482 merged and included in a release, and then fingers crossed we'll be able to merge this PR here!

[davidhewitt]

Many thanks @konstin - CI is all green here using maturin 0.10.0! 🚀

[kngwyu]

Thanks!
Though I didn't fully check the code, I like the renaming, especially rustapi-module. Maybe we can close #1515 after merging this.

[konstin]

Two small things, looks really nice otherwise

[kngwyu]

Thanks!

[kngwyu]

Why do we need this?

[davidhewitt]

This used to be in setup.py, but we now build the tests with maturin instead of setuptools-rust.

We need it because some tests only compile on certain python / PyPy combinations.

IMO this build.rs method is better anyway because it would still work if we changed the examples back to setuptools_rust.

It's a little ugly to have to call a Python interpreter here. I have an idea how to split some of pyo3's build.rs logic into a separate crate pyo3-build-config, which this build.rs could then re-use. I've been torn for a while on whether that refactoring is worth the additional complexity; maybe I should hurry up and push that PR for a while so that others can see what they think.

[davidhewitt]

I'm going to move this into a new issue so that it doesn't block this PR with all the new examples, which I think has value even if there's this this wart.