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.

Python/compile.c

@@ -549,7 +549,7 @@ compiler_enter_scope(struct compiler *c, identifier name,
         return 0;
     }
     if (u->u_ste->ste_needs_class_closure) {
-        /* Cook up a implicit __class__ cell. */
+        /* Cook up an implicit __class__ cell. */
         _Py_IDENTIFIER(__class__);
         PyObject *tuple, *name, *zero;
         int res;