Clarify skip reason

Author: johnslavik

Lib/test/test_traceback.py

@@ -2295,15 +2295,14 @@ def setUp(self) -> None:
 
     def test_unhashable(self):
         raise unittest.SkipTest(
-            "test requires source lines to be captured in tracebacks")
+            "io unavailable (test requires source lines to be captured in tracebacks)")
 
     def test_traceback_format_with_cleared_frames(self):
         raise unittest.SkipTest(
-            "test requires source lines to be captured in tracebacks")
-
+            "io unavailable (test requires source lines to be captured in tracebacks)")
     def test_traceback_format(self):
         raise unittest.SkipTest(
-            "test requires source lines to be captured in tracebacks")
+            "io unavailable (test requires source lines to be captured in tracebacks)")
 
     def tearDown(self) -> None:
         sys.modules['io'] = self.original_io