[v3alpha] `pmb init` fails in addtional options

[17:47:33] Additional options: extra free space: 0 MB, boot partition size: 256 MB, parallel jobs: 9, ccache per arch: 5G, sudo timer: False, mirror: http://mirror.postmarketos.org/postmarketos/
[17:47:33] Change them? (y/n) [n]: y
....
[17:47:46] Enable background timer to prevent repeated sudo authorization? (y/n) [n]: 
[17:47:46] ERROR: 'Config' object has no attribute 'mirrors_postmarketos'
[17:47:46] See also: <https://postmarketos.org/troubleshooting>