#11999: sync based on comparing mtimes, not mtime to system clock

diff --cc Lib/test/test_mailbox.py
Simple merge

diff --cc Misc/NEWS
index 0c9d91a077b24c1bb33b9dc9d984840ce305b7d8,1f48a14d441f53b48da832e35a94d79a7a1fc78c..637e254bb8a5bbf8b0416c89ef7d3434b465ec5c
--- 1/Misc/NEWS
--- 2/Misc/NEWS
+++ b/Misc/NEWS
@@@ -140,12 -83,10 +140,16 @@@ Core and Builtin
  Library
  -------
  
+ - Issue 11999: fixed sporadic sync failure mailbox.Maildir due to its trying to
+   detect mtime changes by comparing to the system clock instead of to the
+   previous value of the mtime.
+ 
 +- Issue #11072: added MLSD command (RFC-3659) support to ftplib.
 +
 +- Issue #8808: The IMAP4_SSL constructor now allows passing an SSLContext
 +  parameter to control parameters of the secure channel.  Patch by Sijin
 +  Joseph.
 +
  - ntpath.samefile failed to notice that "a.txt" and "A.TXT" refer to the same
    file on Windows XP. As noticed in issue #10684.