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/Include/codecs.h b/Include/codecs.h
index 8a9041b..851bc9f 100644
--- a/Include/codecs.h
+++ b/Include/codecs.h
@@ -146,14 +146,14 @@
        const char *encoding
        );
 
-/* Get a IncrementalEncoder object for the given encoding. */
+/* Get an IncrementalEncoder object for the given encoding. */
 
 PyAPI_FUNC(PyObject *) PyCodec_IncrementalEncoder(
        const char *encoding,
        const char *errors
        );
 
-/* Get a IncrementalDecoder object function for the given encoding. */
+/* Get an IncrementalDecoder object function for the given encoding. */
 
 PyAPI_FUNC(PyObject *) PyCodec_IncrementalDecoder(
        const char *encoding,