Mark test__issue6.py flaky everywhere, I'm getting tired of re-running test...

Mark test__issue6.py flaky everywhere, I'm getting tired of re-running test environemnts on Travis one at a time because of it.

greentest/test__issue6.py

@@ -8,6 +8,9 @@ if not sys.argv[1:]:
     code = p.poll()
     assert p.poll() == 0, (out, err, code)
     assert out.strip() == b'11 chars.', (out, err, code)
+    # XXX: This is seen sometimes to fail on Travis with the following value in err but a code of 0;
+    # it seems load related:
+    #  'Unhandled exception in thread started by \nsys.excepthook is missing\nlost sys.stderr\n'
     assert err == b'', (out, err, code)
 
 elif sys.argv[1:] == ['subprocess']:

known_failures.py

@@ -24,6 +24,11 @@ FAILING_TESTS = [
     # Sometimes fails with AssertionError: ...\nIOError: close() called during concurrent operation on the same file object.\n'
     # Sometimes it contains "\nUnhandled exception in thread started by \nsys.excepthook is missing\nlost sys.stderr\n"
     "FLAKY test__subprocess_interrupted.py",
+    # test__issue6 (see comments in test file) is really flaky on both Travis and Appveyor;
+    # on Travis we could just run the test again (but that gets old fast), but on appveyor
+    # we don't have that option without a new commit---and sometimes we really need a build
+    # to succeed in order to get a release wheel
+    'FLAKY test__issue6.py',
 ]
 
 
@@ -60,11 +65,6 @@ if sys.platform == 'win32':
         # on appveyor
         'FLAKY test__timeout.py',
         'FLAKY test_hub_join_timeout.py',
-        # test__issue6 is rarely flaky on both Travis and Appveyor;
-        # on travis we can just run the test again, but on appveyor
-        # we don't have that option without a new commit---and sometimes we really need a build
-        # to succeed in order to get a release wheel
-        'FLAKY test__issue6.py',
     ]
 
     if struct.calcsize('P') * 8 == 64: