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
or edited some other way to fix the grammar.
diff --git a/Doc/library/http.server.rst b/Doc/library/http.server.rst
index 1c3e202..0bde35b 100644
--- a/Doc/library/http.server.rst
+++ b/Doc/library/http.server.rst
@@ -176,7 +176,7 @@
 
    .. method:: handle_expect_100()
 
-      When a HTTP/1.1 compliant server receives a ``Expect: 100-continue``
+      When a HTTP/1.1 compliant server receives an ``Expect: 100-continue``
       request header it responds back with a ``100 Continue`` followed by ``200
       OK`` headers.
       This method can be overridden to raise an error if the server does not
@@ -210,7 +210,7 @@
       are picked up from the :meth:`version_string` and
       :meth:`date_time_string` methods, respectively. If the server does not
       intend to send any other headers using the :meth:`send_header` method,
-      then :meth:`send_response` should be followed by a :meth:`end_headers`
+      then :meth:`send_response` should be followed by an :meth:`end_headers`
       call.
 
       .. versionchanged:: 3.3