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

This affects documentation, code comments, and a debugging messages.

Python/codecs.c

@@ -403,7 +403,7 @@ wrap_codec_error(const char *operation,
                            operation, encoding);
 }
 
-/* Encode an object (e.g. an Unicode object) using the given encoding
+/* Encode an object (e.g. a Unicode object) using the given encoding
    and return the resulting encoded object (usually a Python string).
 
    errors is passed to the encoder factory as argument if non-NULL. */
@@ -450,7 +450,7 @@ _PyCodec_EncodeInternal(PyObject *object,
 }
 
 /* Decode an object (usually a Python string) using the given encoding
-   and return an equivalent object (e.g. an Unicode object).
+   and return an equivalent object (e.g. a Unicode object).
 
    errors is passed to the decoder factory as argument if non-NULL. */