The mailboxes must be a tuple of string or the string "ALL".
The generated value was broken if the mailboxes configuration was a list
of only one string (but not "ALL"): the generated expression ( "str" )
was not a tuple but a string.
Now, we always generate a tuple (by adding a comma, even with a list of
size one). Getmail works with the special value "ALL" whether it is a
in tuple or not, so this case is not specifically handled.
