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.

Objects/exceptions.c

@@ -867,7 +867,7 @@ OSError_init(PyOSErrorObject *self, PyObject *args, PyObject *kwds);
 static int
 oserror_use_init(PyTypeObject *type)
 {
-    /* When __init__ is defined in a OSError subclass, we want any
+    /* When __init__ is defined in an OSError subclass, we want any
        extraneous argument to __new__ to be ignored.  The only reasonable
        solution, given __new__ takes a variable number of arguments,
        is to defer arg parsing and initialization to __init__.