Fix casting ssize_t to unsigned

Cython/Utility/Buffer.c

@@ -200,7 +200,7 @@ static int __Pyx__GetBufferAndValidate(
     __Pyx_BufFmt_Init(&ctx, stack, dtype);
     if (!__Pyx_BufFmt_CheckString(&ctx, buf->format)) goto fail;
   }
-  if (unlikely((unsigned)buf->itemsize != dtype->size)) {
+  if (unlikely((size_t)buf->itemsize != dtype->size)) {
     PyErr_Format(PyExc_ValueError,
       "Item size of buffer (%" CYTHON_FORMAT_SSIZE_T "d byte%s) does not match size of '%s' (%" CYTHON_FORMAT_SSIZE_T "d byte%s)",
       buf->itemsize, (buf->itemsize > 1) ? "s" : "",