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/Lib/io.py b/Lib/io.py
index 8d68f1e..e03db97 100644
--- a/Lib/io.py
+++ b/Lib/io.py
@@ -19,7 +19,7 @@
Another IOBase subclass, TextIOBase, deals with the encoding and decoding
of streams into text. TextIOWrapper, which extends it, is a buffered text
interface to a buffered raw stream (`BufferedIOBase`). Finally, StringIO
-is a in-memory stream for text.
+is an in-memory stream for text.
Argument names are not part of the specification, and only the arguments
of open() are intended to be used as keyword arguments.