This allows *from_* to be successfully set to a non-default value when calling mbox.get_string.

in the mailbox module.  This is partial rollback of changeset f340cb045bf9.

Backported from patch by Jeremy Kloth.

Patch by Jeremy Kloth.

Original patch by Matti Picus.

If messages were only added, a new file is no longer created and
renamed over the old file when flush() is called on an mbox, MMDF or
Babyl mailbox.

If messages were only added, a new file is no longer created and
renamed over the old file when flush() is called on an mbox, MMDF or
Babyl mailbox.

Previously mailbox was copying a list of attributes from one message object to
another in order to "copy the message data".  This means that any time new
attributes were added to email.message.Message, mailbox broke.  Now instead it
copies all attributes from the source object to the target object, skipping
any mailbox-object-specific attributes to produce the same clean initial
state it was previously getting by copying only the "known" attributes.

David Lam assisted in the development of this patch.

In Python2, if a unicode string was assigned as the value of a header,
email would automatically CTE encode it using the UTF8 charset.
This capability was lost in the Python3 translation, and this patch
restores it.

Patch by Ali Ikinci, assisted by R. David Murray.

I also added a fix for the mailbox test that was depending (with a comment
that it was a bad idea to so depend) on non-ASCII causing message_from_string
to raise an error.  It now uses support.patch to induce an error during
message serialization.

Closes #14158: We now track test_support.TESTFN cleanup, and test_mailbox uses shutil.rmtree for simpler code.

support multiprocessing.Event.

support multiprocessing.Event.

race condition).

race condition).