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/Python/compile.c b/Python/compile.c
index 686510c..ff62487 100644
--- a/Python/compile.c
+++ b/Python/compile.c
@@ -549,7 +549,7 @@
         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;
diff --git a/Python/thread.c b/Python/thread.c
index 4e695ae..810691f 100644
--- a/Python/thread.c
+++ b/Python/thread.c
@@ -31,7 +31,7 @@
    threads.
 
    This is valid for HP-UX 11.23 running on an ia64 system. If needed, add
-   a check of __ia64 to verify that we're running on a ia64 system instead
+   a check of __ia64 to verify that we're running on an ia64 system instead
    of a pa-risc system.
 */
 #ifdef __hpux