gh-132952: Import _io instead of io at startup

[JelleZijlstra]

• Issue: #127648 introduced a ~12% performance regression in python_startup_no_site benchmark #132952

[JelleZijlstra]

Like #132956 this only helps -S startup because io gets imported anyway by site.py and possibly other places. Maybe we can make those other places also use _io but not convinced it's worth it.

[mdboom]

I'm going to run this branch on our benchmarking hardware (as well as #132956), just so we have some numbers.

[mdboom]

Benchmarking of this PR: https://github.com/faster-cpython/benchmarking-public/blob/main/results/bm-20250425-3.14.0a7%2B-dac50cc/bm-20250425-linux-x86_64-JelleZijlstra-sunder_io-3.14.0a7%2B-dac50cc-vs-base.svg

This is actually more effective than (19%) than #132956 (12%). I think it makes sense to merge this, and maybe the other just for good measure (though it should have no additional effect).

[JelleZijlstra]

This is now ready.

One tricky aspect is that we need to check any side effects that come from importing io. Most of the file is defining ABCs and registering things to those ABCs; it's fine if we omit that because the ABCs don't exist if the module isn't imported. However, it also sets the __module__ attribute on the UnsupportedOperation exception. I changed the code to set that attribute in _io instead because otherwise users who never import io would see this exception with the wrong module name.

[gpshead]

it is worth mentioning "vs what" when stating an improvement %.

[mdboom]

It's 19% vs. before this commit, but it's only 1.5% faster than 3.13.0 (which is probably what most users care about, since they don't run main). I would argue 1.5% is in the noise, so we probably shouldn't have a what's new entry at all -- this is just counteracting a regression in 3.14 development.

[JelleZijlstra]

It's only a NEWS entry, not text in the What's New, so I'll remove the 19% claim but leave the NEWS entry since this could affect behavior some users care about.

[mdboom]

LGTM, but I think we should remove the whatsnew.

[bedevere-app]

When you're done making the requested changes, leave the comment: I have made the requested changes; please review again.

[JelleZijlstra]

I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@mdboom, @gpshead: please review the changes made to this pull request.