Support for PyO3 0.23

[ngoldbaum]

I'm looking at using rust-numpy with the free-threaded Python build and it looks like the first blocker is 0.23 support. The error I hit out of the box after pointing the cargo.toml at my local copy of PyO3 is:

error: failed to select a version for `pyo3`.
    ... required by package `numpy v0.21.0 (/Users/goldbaum/Documents/rust-numpy)`
versions that meet the requirements `*` are: 0.23.0-dev

the package `numpy` depends on `pyo3`, with features: `gil-refs` but `pyo3` does not have these features.

So it looks like the first step is removing the uses of the gil-refs feature and fully transitioning to the bound reference API.

[davidhewitt]

Yep, looks like @Icxolu has made great progress in #457 (🙏). We wouldn't merge that until we actually release 0.23 upstream, however.

[kylebarron]

Now that #457 has been merged, would we be able to get an 0.23 release of numpy?

[davidhewitt]

I hope to prep a release branch tonight :)

[davidhewitt]

#473 is in the oven :)