X509 serial numbers are bigger than C ints (or even C
long long). A Python long should be used to get/set
them, not a Python int. I wrote a patch for the
get_serial_number method, but not (yet) for the
set_serial method (I didn't need it, and since there is
no PyLong_ToString function in the Python/C API, it'll
be a bit more tricky).