gh-143768: Replace a dangling interpreter symlink when creating a venv

[claydugo]

A dangling interpreter symlink in the target directory caused venv to crash with --copies and to silently leave the broken link in place with symlinks.
symlink_or_copy now unlinks a dangling destination symlink first, matching how a stale regular file is already replaced on re-run.

Supersedes #143770, which got stuck in a wedged change-detection state after a bad merge via the GitHub UI.

• Issue: venv fails with confusing ENOENT when existing venv has broken symlinks #143768

[brettcannon]

I just realized that there's no doc update about the semantic change. Otherwise LGTM!

[bedevere-app]

A Python core developer has requested some changes be made to your pull request before we can consider merging it. If you could please address their requests along with any other requests in other reviews from core developers that would be appreciated.

Once you have made the requested changes, please leave a comment on this pull request containing the phrase I have made the requested changes; please review again. I will then notify any core developers who have left a review that you're ready for them to take another look at this pull request.

[claydugo]

Thanks for the review! Just pushed a doc update

[read-the-docs-community]

Documentation build overview

📚 cpython-previews | 🛠️ Build #33972991 | 📁 Comparing dc59676 against main (8ed1479)

  🔍 Preview build  

2 files changed

± library/venv.html
± whatsnew/changelog.html

[brettcannon]

Thanks for the review! Just pushed a doc update

If you're ready for another review then don't forget to follow the instructions from the bot about how to notify folks you're ready.

[claydugo]

Oops, glazed right over that. Sorry. I have made the requested changes; please review again

[bedevere-app]

Thanks for making the requested changes!

@brettcannon: please review the changes made to this pull request.