Normalize the encoding names for Latin-1 and UTF-8 to
'latin-1' and 'utf-8'.

These are optimized in the Python Unicode implementation
to result in more direct processing, bypassing the codec
registry.

Also see issue11303.
diff --git a/Lib/test/test_subprocess.py b/Lib/test/test_subprocess.py
index 4b58308..9e267eb 100644
--- a/Lib/test/test_subprocess.py
+++ b/Lib/test/test_subprocess.py
@@ -1191,7 +1191,7 @@
         stdout, stderr = p.communicate()
         self.assertEqual(0, p.returncode, "sigchild_ignore.py exited"
                          " non-zero with this error:\n%s" %
-                         stderr.decode('utf8'))
+                         stderr.decode('utf-8'))
 
 
 @unittest.skipUnless(mswindows, "Windows specific tests")