Fix memory leak in PLySequence_ToJsonbValue()

Fix memory leak in PLySequence_ToJsonbValue()

PyObject returned from PySequence_GetItem() is not released.  Similar code in PLyMapping_ToJsonbValue() is correct, because according to Python documentation
PyList_GetItem() and PyTuple_GetItem() return a borrowed reference while
PySequence_GetItem() returns new reference.  contrib/jsonb_plpython is new
in PostgreSQL 11, no backpatch is needed.

Author: Nikita Glukhov
Discussion: https://postgr.es/m/6001af16-b242-2527-bc7e-84b8a959163b%40postgrespro.ru