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

This affects documentation and code comments.
diff --git a/Modules/_elementtree.c b/Modules/_elementtree.c
index 0d16a69..6f628a5 100644
--- a/Modules/_elementtree.c
+++ b/Modules/_elementtree.c
@@ -2104,7 +2104,7 @@
         /* decode universal name */
 #if defined(Py_USING_UNICODE)
         /* inline makestring, to avoid duplicating the source string if
-           it's not an utf-8 string */
+           it's not a utf-8 string */
         p = PyString_AS_STRING(tag);
         if (checkstring(p, size)) {
             value = PyUnicode_DecodeUTF8(p, size, "strict");