bpo-41540: AIX: skip test that is flaky with a default ulimit. (GH-21890) (#21893)

- AIX has extreme over-allocation that is in no relation to the physical
    RAM and swap.

(cherry picked from commit 39dab24621)

Authored-by: Stefan Krah <skrah@bytereef.org>

Lib/test/test_decimal.py

@@ -5478,6 +5478,9 @@ class CWhitebox(unittest.TestCase):
             self.assertEqual(Decimal.from_float(cls(101.1)),
                              Decimal.from_float(101.1))
 
+    # Issue 41540:
+    @unittest.skipIf(sys.platform.startswith("aix"),
+                     "AIX: default ulimit: test is flaky because of extreme over-allocation")
     def test_maxcontext_exact_arith(self):
 
         # Make sure that exact operations do not raise MemoryError due