refactor: Rewrite logic to get first element of mount dirs

Using `next(iter(...))` is equally elegant as `.pop`. However, it will
work with mappings and sets, proactively fixing a bug that would
otherwise come up when updating storge-device-manager.

src/butter_backup/cli.py

@@ -108,7 +108,7 @@ def close(config: Path = CONFIG_OPTION, verbose: int = VERBOSITY_OPTION) -> None
                 raise ValueError(
                     "Got several possible mount points. Expected exactly 1!"
                 )
-            mount_dir = mount_dirs.pop()
+            mount_dir = next(iter(mount_dirs))
             sdm.unmount_device(mount_dir)
             sdm.close_decrypted_device(Path(mapped_device))
             mount_dir.rmdir()