Pre-load script wrapper code to avoid errors when pip is being upgraded

[pfmoore]

Fixes #12666

[pradyunsg]

@pfmoore Have you considered filing a PR against distlib for this?

[pfmoore]

I assumed that it wasn't worth it - it uses extra memory and adds an import-time performance hit, simply to protect against a situation that should never come up in normal code (modifying site-packages while using packages imported from it is really dodgy, unless you're an installer like pip, and even then it's perilous). But maybe distlib would be OK with it anyway.

I've created a PR for this - pypa/distlib#215 - we can see what happens.

Is it worth merging this anyway, on the assumption that we won't get a new version of distlib before 24.1? It's not a major problem, but it would be nice to get a fix in now we know what's been causing it all this time 🙂

[pfmoore]

Fix has been merged into distlib and will be released in their next scheduled release (mid-year). I declined their offer of an interim release as I don't think this is urgent enough to warrant it.

Should we merge this as a temporary fix, or drop it and leave the issue until the distlib release? I'm inclined to merge - we have a fix and removing the patch when distlib gets released isn't hard - but @pradyunsg if you prefer not to, that's fine.

[pradyunsg]

Let's go ahead and merge this -- I think it's suboptimal that we have to take in an upstream patch for this but this is a piece of technical debt that we'd pay down on the next upgrade of distlib and it shouldn't really affect non-Windows users (who I expect won't be debundling pip for redistribution).