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/Doc/library/sunau.rst b/Doc/library/sunau.rst
index 4bdb99b..5d57d4b 100644
--- a/Doc/library/sunau.rst
+++ b/Doc/library/sunau.rst
@@ -54,8 +54,8 @@
Note that it does not allow read/write files.
- A *mode* of ``'r'`` returns a :class:`AU_read` object, while a *mode* of ``'w'``
- or ``'wb'`` returns a :class:`AU_write` object.
+ A *mode* of ``'r'`` returns an :class:`AU_read` object, while a *mode* of ``'w'``
+ or ``'wb'`` returns an :class:`AU_write` object.
.. function:: openfp(file, mode)