fix(pypi): fallback to simple lookup on 403

[Kakadus]

Changes

This adds 403 to the simple index fallback check, so that pytorch index urls are correctly resolved by renovate.

Context

See #26383

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[viceice]

🤔 this can cause a lot confusion when people try to get authentication to work with custom pypi registries.

[sorgfresser]

Right now the PyTorch dependency lookup fails with their private registry - while this might be an issue with torch's behaviour, renovate could fix this by defaulting to simple on 403s. I get that this could be confusing though.

Could this retry be achieved using hostRules in the renovate config defaulting to the old behaviour, i.e. no retry? This way we could fix the PyTorch-issue while still not confusing the setup of private registries.

[rarkins]

Can we just restrict it to the pytorch domain for now?

[rarkins]

This would be a harder but better long term solution: #26483

[Kakadus]

I've tried to implement this in #26505. Closing this in favor of it for now.