maturin examples

[kngwyu]

Now our examples are built by setuptools-rust, but since currently maturin is (maybe more) widely used, we should also provide maturin examples.

[davidhewitt]

As I'm working on the experimental maturin support for #1040 I'm going to convert the existing examples to maturin soon, probably this week.

I'll add a new minimalist example showing setuptools-rust configuration.

[konstin]

You should be able to just leave the setuptools part in since maturin simply ignores those files, so that the setuptools fans still get an example.

[davidhewitt]

Having started playing around with this, I found that it's nicer to make separate maturin and setuptools-rust examples because then it's easier for users to see what configuration is needed depending on the packaging tool they chose.

I ran into a bit of a speedbump with the tox testing of the maturin example. You've seen already the PRs I put into tox and maturin to resolve this, though I still have issues because the PEP 517 out-of-tree build fails because the pyo3 path = "../../" obviously doesn't get found when building in a temporary directory.

I'm trying to decide if I should hack in a wrapper around maturin as a PEP 517 "inline backend" to solve this issue, or whether to give up and just test the setuptools-rust bit using tox.

Hopefully I can find some time to have another go at finishing this off soon.