config: Formatting
config: Raise ValueError if Path is asked to serialize unicode

If we accept unicode and try to encode using sys.getfilesystemencoding() then
it may work most of the time, but will fail if we get non-ASCII chars in the
unicode string and the file system encoding is e.g. ANSI-something because the
locale is C. Thus, I figure it is better to always fail if we try to serialize
Path from unicode strings. Paths should be maintained as bytes all the time.