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/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.
                 */