Issue #25523: Correct "a" article to "an" article

This changes the main documentation, doc strings, source code comments, and a
couple error messages in the test suite. In some cases the word was removed
to fix the grammar.
diff --git a/Lib/idlelib/EditorWindow.py b/Lib/idlelib/EditorWindow.py
index 0c5b713..8a33719 100644
--- a/Lib/idlelib/EditorWindow.py
+++ b/Lib/idlelib/EditorWindow.py
@@ -1384,7 +1384,7 @@
             text.see("insert")
             text.undo_block_stop()
 
-    # Our editwin provides a is_char_in_string function that works
+    # Our editwin provides an is_char_in_string function that works
     # with a Tk text index, but PyParse only knows about offsets into
     # a string. This builds a function for PyParse that accepts an
     # offset.