[PATCH] contrib/importers/keepass2csv2pass.py: remove deprecated 'U' open mode

[a0ddca20-0666-4cad-b8a8-da73c13955da at simplelogin.com]

Looks like the guy posting this forgot to quote the original email. So
I searched for it and found the original email archived at
https://lists.zx2c4.com/pipermail/password-store/2023-April/004749.html

> Having also encountered this issue recently, I was about to submit a patch myself until I saw this. I'm bumping this thread so that it has a chance of being looked at again and perhaps accepted.