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/PC/msvcrtmodule.c b/PC/msvcrtmodule.c
index 057900d..44c82e4 100644
--- a/PC/msvcrtmodule.c
+++ b/PC/msvcrtmodule.c
@@ -148,7 +148,7 @@
     if (handle == -1)
         return PyErr_SetFromErrno(PyExc_IOError);
 
-    /* technically 'handle' is not a pointer, but a integer as
+    /* technically 'handle' is not a pointer, but an integer as
        large as a pointer, Python's *VoidPtr interface is the
        most appropriate here */
     return PyLong_FromVoidPtr((void*)handle);