Issue #12848: The pure Python pickle implementation now treats object lengths as unsigned 32-bit integers, like the C implementation does.

Patch by Serhiy Storchaka.

Modules/_pickle.c

@@ -1589,7 +1589,7 @@ save_long(PicklerObject *self, PyObject *obj)
          * byte at the start, and cut it back later if possible.
          */
         nbytes = (nbits >> 3) + 1;
-        if (nbytes > INT_MAX) {
+        if (nbytes > 0x7fffffffL) {
             PyErr_SetString(PyExc_OverflowError,
                             "long too large to pickle");
             goto error;