Module cleanup function name not mangled

[robertwb]

Cython generates this code these days:

static int __Pyx_RegisterCleanup(); /*proto*/
static PyObject* cleanup(PyObject *self, PyObject *unused); /*proto*/
static PyMethodDef cleanup_def = {"__cleanup", (PyCFunction)&cleanup,
METH_NOARGS, 0};

The "cleanup" function should be mangled consistently with how all
other Cython auto-generated
code is mangled. Otherwise a user could easily get really confused if
they say using a C library
that defines a function called cleanup.