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

This affects documentation and code comments.
diff --git a/Lib/test/test_print.py b/Lib/test/test_print.py
index b726347..e8d9e66 100644
--- a/Lib/test/test_print.py
+++ b/Lib/test/test_print.py
@@ -107,7 +107,7 @@
         self.assertRaises(AttributeError, print, '', file='')
 
     def test_mixed_args(self):
-        # If an unicode arg is passed, sep and end should be unicode, too.
+        # If a unicode arg is passed, sep and end should be unicode, too.
         class Recorder(object):
 
             def __init__(self, must_be_unicode):