closes bpo-35623: Fix integer overflow when sorting large lists (GH-11380)

There is already a `Py_ssize_t i` defined at function scope that is used
for similar loops. By removing the local `int i` declaration that `i` is
used, which has the appropriate type.
(cherry picked from commit f8b5344)

Co-authored-by: sth <[email protected]>

Author: miss-islington

Misc/NEWS.d/next/Core and Builtins/2018-12-31-02-37-20.bpo-35623.24AQhY.rst

@@ -0,0 +1 @@
+Fix a crash when sorting very long lists. Patch by Stephan Hohe.

Objects/listobject.c

@@ -2232,7 +2232,6 @@ list_sort_impl(PyListObject *self, PyObject *keyfunc, int reverse)
         int ints_are_bounded = 1;
 
         /* Prove that assumption by checking every key. */
-        int i;
         for (i=0; i < saved_ob_size; i++) {
 
             if (keys_are_in_tuples &&