Add pyodide_315 support

[hoodmane]

Guarded by cpython_prerelease. I manually tested building pywavelets and it works as expected.

• Fix cross build environment installation not to try to install native scipy

[henryiii]

Quick background:

We went with cpython_prerelease because this is supposed to be ABI stable from pyodide's perspective for the new pyemscripten release. We could drop pyodide_prerelease if there's not really a reason for it anymore.

We want to get this in before the 3.15rc1 release because numpy won't add pyemcripten as a platform unless the can add it for both CPython 3.15 and pyemscripten 3.15 in the same numpy release. (Seems like a rather odd requirement, and patch releases have added things before, but getting pyodide in sync with CPython is nice anyway).

[agriyakhetarpal]

Drive-by comment for now: I'd definitely want pyodide-prerelease group to stay, instead of Pyodide releases being enabled with cpython-prerelease; that is a clearer separation of concerns

[hoodmane]

@agriyakhetarpal I don't see why pyodide_prerelease should stay, the goal is to have it at the same time as the cpython prerelease.

[agriyakhetarpal]

@hoodmane it conflates CPython and Pyodide. If someone enables cpython-prerelease with the changes here and also has a Pyodide build workflow, I'd lean toward not starting to build pyemscripten alpha ABI wheels. There's a reason we have Pyodide as a separate platform, after all, rather than mixing it with GraalPy.

[hoodmane]

Pyodide is CPython so it's correct to conflate them.

[hoodmane]

I think if someone is building both Pyodide releases and cpython prereleases it's desirable that that also builds Pyodide for prerelease Python versions. I don't see how that would be a bad thing. If they don't want it they can explicitly disable pre-release Pyodide. I think people won't remember to explicitly enable pre-release Pyodide so it's much better as opt-out than opt-in.

[ryanking13]

I am in favor of cpython_prerelease change with two reasons:

1. We are aiming to release Pyodide prerelease at the same time of CPython rc from this release
2. Pyodide cibuildwheel should be configured separately from other platforms (at least at this point), so I don't think people will accidentally enable Pyodide builds when they do not want.

Well, all three of you are in favour and I don't have veto powers, so... :D

[joerick]

No test regressions with the alpha is very encouraging :) I'm okay with using cpython-prerelease for this, as long as we're expecting the timelines between pyodide and cpython to be the same.