Correct “an” → “a” with “Unicode”, “user”, “UTF”, etc

This affects documentation and code comments.
diff --git a/Lib/test/test_zlib.py b/Lib/test/test_zlib.py
index c8fc985..cc13409 100644
--- a/Lib/test/test_zlib.py
+++ b/Lib/test/test_zlib.py
@@ -150,7 +150,7 @@
         self.assertEqual(zlib.decompress(x), data)
 
     def test_incomplete_stream(self):
-        # An useful error message is given
+        # A useful error message is given
         x = zlib.compress(HAMLET_SCENE)
         self.assertRaisesRegexp(zlib.error,
             "Error -5 while decompressing data: incomplete or truncated stream",