Issue 21360: mailbox.Maildir should ignore files named with a leading dot

The maildir format specification
(see http://cr.yp.to/proto/maildir.html) is clear that files named with leading dots should be ignore:

  Unless you're writing messages to a maildir, the format of a unique 
  name is none of your business. A unique name can be anything that 
  doesn't contain a colon (or slash) and doesn't start with a dot. Do not
  try to extract information from unique names.

Test case:

liw@havelock$ find Maildir -ls
8921206    4 drwxrwxr-x   5 liw      liw          4096 Apr 26 23:03 Maildir
8921207    4 drwxrwxr-x   2 liw      liw          4096 Apr 26 23:03 Maildir/cur
8921209    4 drwxrwxr-x   2 liw      liw          4096 Apr 26 23:03 Maildir/tmp
8921208    4 drwxrwxr-x   2 liw      liw          4096 Apr 26 23:03 Maildir/new
8913523    0 -rw-rw-r--   1 liw      liw             0 Apr 26 23:03 Maildir/new/.foo
liw@havelock$ python -c 'import mailbox
maildir = mailbox.Maildir("Maildir")
print maildir.keys()
'
['.foo']
liw@havelock$ 

The correct output would be the empty list.

"mutt -f Maildir" correctly shows now messages in that folder.

I wonder if changing this would break any working programs.  I'd like to think not, but it is certainly possible for someone to be actually depending this bug.

I've created a PR for this issue.

David: How do you propose we move forward on this? 

Should we provide a switch to keep the original behaviour (which is off by default) so if there is someone depending on this bug, they could still continue to do it with relatively small change?

It is a change of behaviour, so probably isn't going to be back ported if it gets merged in 3.9 window.

messages: + msg335375
versions: + Python 3.7, Python 3.8