Fix use of test skipping API.

Lib/test/test_io.py

@@ -509,11 +509,11 @@ class IOTest(unittest.TestCase):
         # Issue #1174606: reading from an unbounded stream such as /dev/zero.
         zero = "/dev/zero"
         if not os.path.exists(zero):
-            self.skip("{0} does not exist".format(zero))
+            self.skipTest("{0} does not exist".format(zero))
         if sys.maxsize > 0x7FFFFFFF:
-            self.skip("test can only run in a 32-bit address space")
+            self.skipTest("test can only run in a 32-bit address space")
         if support.real_max_memuse < support._2G:
-            self.skip("test requires at least 2GB of memory")
+            self.skipTest("test requires at least 2GB of memory")
         with open(zero, "rb", buffering=0) as f:
             self.assertRaises(OverflowError, f.read)
         with open(zero, "rb") as f: