Implement foldersort in a python3 compatible way

By default we sort folders alphabetically (for IMAP) according to their
transposed names. For python3, we need to bend a bit backwards to still
allow the use of a cmp() function for foldersort. While going through, I
discovered that we never sort folders for Maildir.

Signed-off-by: Sebastian Spaeth <Sebastian@SSpaeth.de>

offlineimap/repository/Base.py

@@ -45,7 +45,7 @@ class BaseRepository(CustomConfig.ConfigHelperMixin, object):
         self.nametrans = lambda foldername: foldername
         self.folderfilter = lambda foldername: 1
         self.folderincludes = []
-        self.foldersort = cmp
+        self.foldersort = None
         if self.config.has_option(self.getsection(), 'nametrans'):
             self.nametrans = self.localeval.eval(
                 self.getconf('nametrans'), {'re': re})