fix: replace rogue `print` statement by `self.ui.debug`

Signed-off-by: Valentin Lab <valentin.lab@kalysto.org>
Signed-off-by: Nicolas Sebrecht <nicolas.s-dev@laposte.net>

offlineimap/init.py

@@ -299,7 +299,7 @@ class OfflineImap:
                 stack_display.append('  => Stopped to handle current signal. ')
             stack_displays.append(stack_display)
         stacks = unique_count(stack_displays)
-        print("** Thread List:\n")
+        self.ui.debug('thread', "** Thread List:\n")
         for stack, times in stacks:
             if times == 1:
                 msg = "%s Thread is at:\n%s\n"