MNT: re-add .txt file extensions

[neutrinoceros]

Dependabot has been unable to handle lock file updates since we added uv.lock. This is caused by a very simple (yet, unadressed) upstream bug:
dependabot/dependabot-core#13960

I naively assumed it would be fixed quickly and concluded it wasn't worth working around, but I just ended up having to handle upgrades manually, which is more error prone and more of a security risk (not to mention extremely boring). I'm going to open a PR removing all .txt extensions just for this reason, but this will serve as a reminder to revert that PR once it's not needed anymore.

[neutrinoceros]

the upstream issue was closed but there's no rush in addressing this, and it'll be less likely we find a new problem if we wait it out a bit longer.