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/smtplib.rst b/Doc/library/smtplib.rst
index 5b2808d..b6e7689 100644
--- a/Doc/library/smtplib.rst
+++ b/Doc/library/smtplib.rst
@@ -22,7 +22,7 @@
 
 .. class:: SMTP([host[, port[, local_hostname[, timeout]]]])
 
-   A :class:`SMTP` instance encapsulates an SMTP connection.  It has methods
+   An :class:`SMTP` instance encapsulates an SMTP connection.  It has methods
    that support a full repertoire of SMTP and ESMTP operations. If the optional
    host and port parameters are given, the SMTP :meth:`connect` method is
    called with those parameters during initialization.  If specified,
@@ -45,7 +45,7 @@
 
 .. class:: SMTP_SSL([host[, port[, local_hostname[, keyfile[, certfile[, timeout]]]]]])
 
-   A :class:`SMTP_SSL` instance behaves exactly the same as instances of
+   An :class:`SMTP_SSL` instance behaves exactly the same as instances of
    :class:`SMTP`. :class:`SMTP_SSL` should be used for situations where SSL is
    required from the beginning of the connection and using :meth:`starttls` is
    not appropriate. If *host* is not specified, the local host is used. If