plpython: add missing plpython.h include to plpy_plpymodule.h
The include was missing before
9b7e24a2cb3, but starting with that commit the
missing include causes cpluspluscheck to fail because the use of
PyMODINIT_FUNC isn't incidentally protected by an ifdef anymore.
Discussion: https://postgr.es/m/
20220308045916.7baapelbgftoqeop@alap3.anarazel.de