Try to make this test more resistant to dropping from previous runs (ie, files that may exist but cause the test to fail). Should be backported (assuming it works :-)

Lib/test/test_mailbox.py

@@ -54,6 +54,7 @@ class TestMailbox(TestBase):
 
     def setUp(self):
         self._path = test_support.TESTFN
+        self._delete_recursively(self._path)
         self._box = self._factory(self._path)
 
     def tearDown(self):