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/concurrent.futures.rst b/Doc/library/concurrent.futures.rst
index cc8b335..e63e741 100644
--- a/Doc/library/concurrent.futures.rst
+++ b/Doc/library/concurrent.futures.rst
@@ -81,7 +81,7 @@
ThreadPoolExecutor
------------------
-:class:`ThreadPoolExecutor` is a :class:`Executor` subclass that uses a pool of
+:class:`ThreadPoolExecutor` is an :class:`Executor` subclass that uses a pool of
threads to execute calls asynchronously.
Deadlocks can occur when the callable associated with a :class:`Future` waits on
@@ -285,7 +285,7 @@
Added callables are called in the order that they were added and are
always called in a thread belonging to the process that added them. If
- the callable raises a :exc:`Exception` subclass, it will be logged and
+ the callable raises an :exc:`Exception` subclass, it will be logged and
ignored. If the callable raises a :exc:`BaseException` subclass, the
behavior is undefined.