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

This affects documentation and code comments.
diff --git a/Misc/HISTORY b/Misc/HISTORY
index b9e74da..60635fb 100644
--- a/Misc/HISTORY
+++ b/Misc/HISTORY
@@ -8024,7 +8024,7 @@
 
 - There is a new Unicode companion to the PyObject_Str() API
   called PyObject_Unicode(). It behaves in the same way as the
-  former, but assures that the returned value is an Unicode object
+  former, but assures that the returned value is a Unicode object
   (applying the usual coercion if necessary).
 
 - The comparison operators support "rich comparison overloading" (PEP