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/Modules/_io/_iomodule.c b/Modules/_io/_iomodule.c
index 04c4445..bbfea71 100644
--- a/Modules/_io/_iomodule.c
+++ b/Modules/_io/_iomodule.c
@@ -74,7 +74,7 @@
 "Another IOBase subclass, TextIOBase, deals with the encoding and decoding\n"
 "of streams into text. TextIOWrapper, which extends it, is a buffered text\n"
 "interface to a buffered raw stream (`BufferedIOBase`). Finally, StringIO\n"
-"is a in-memory stream for text.\n"
+"is an in-memory stream for text.\n"
 "\n"
 "Argument names are not part of the specification, and only the arguments\n"
 "of open() are intended to be used as keyword arguments.\n"
diff --git a/Modules/_io/iobase.c b/Modules/_io/iobase.c
index 710ada4..5f7b345 100644
--- a/Modules/_io/iobase.c
+++ b/Modules/_io/iobase.c
@@ -35,7 +35,7 @@
     "Even though IOBase does not declare read, readinto, or write because\n"
     "their signatures will vary, implementations and clients should\n"
     "consider those methods part of the interface. Also, implementations\n"
-    "may raise a IOError when operations they do not support are called.\n"
+    "may raise an IOError when operations they do not support are called.\n"
     "\n"
     "The basic type used for binary data read from or written to a file is\n"
     "the bytes type. Method arguments may also be bytearray or memoryview\n"
diff --git a/Modules/_io/textio.c b/Modules/_io/textio.c
index 38bb0d8..a95edce 100644
--- a/Modules/_io/textio.c
+++ b/Modules/_io/textio.c
@@ -881,8 +881,8 @@
             if (self->encoding == NULL) {
               catch_ImportError:
                 /*
-                 Importing locale can raise a ImportError because of
-                 _functools, and locale.getpreferredencoding can raise a
+                 Importing locale can raise an ImportError because of
+                 _functools, and locale.getpreferredencoding can raise an
                  ImportError if _locale is not available.  These will happen
                  during module building.
                 */