[3.14] gh-63161: Fix test_source_encoding when stderr is StringIO (GH-140361)

Author: serhiy-storchaka

Lib/test/test_source_encoding.py

@@ -521,7 +521,7 @@ def check_script_error(self, src, expected, lineno=...):
             line = src.splitlines()[lineno-1].decode(errors='replace')
             if lineno == 1:
                 line = line.removeprefix('\ufeff')
-            line = line.encode(sys.stderr.encoding, sys.stderr.errors)
+            line = line.encode(sys.__stderr__.encoding, sys.__stderr__.errors)
             self.assertIn(line, err)