alot: allow commas in maildir path (#6989)

Previously, maildir path was not escaped in any way, so using a path with commas
lead to a TypeError:

```
TypeError: expected string or bytes-like object, got 'list'
```

tests/modules/programs/alot/alot-expected.conf

@@ -26,10 +26,10 @@ prefer_plaintext = True
 
 [[hm@example.com]]
 address=hm@example.com
-draft_box=maildir:///home/hm-user/Mail/hm@example.com/Drafts
+draft_box='maildir:///home/hm-user/Mail/hm@example.com/Drafts'
 realname=H. M. Test
 sendmail_command=
-sent_box=maildir:///home/hm-user/Mail/hm@example.com/Sent
+sent_box='maildir:///home/hm-user/Mail/hm@example.com/Sent'
 auto_remove_unread = True
 ask_subject = False
 handle_mouse = True