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

This affects documentation and code comments.
diff --git a/Include/unicodeobject.h b/Include/unicodeobject.h
index d15ebb8..9184eb1 100644
--- a/Include/unicodeobject.h
+++ b/Include/unicodeobject.h
@@ -513,7 +513,7 @@
     Py_ssize_t length           /* New length */
     );
 
-/* Coerce obj to an Unicode object and return a reference with
+/* Coerce obj to a Unicode object and return a reference with
    *incremented* refcount.
 
    Coercion is done in the following way:
@@ -536,7 +536,7 @@
     const char *errors          /* error handling */
     );
 
-/* Coerce obj to an Unicode object and return a reference with
+/* Coerce obj to a Unicode object and return a reference with
    *incremented* refcount.
 
    Unicode objects are passed back as-is (subclasses are converted to