/usr/include/python2.7/mpi4py is in python-mpi4py 2.0.0-2.1+deb9u1.
This file is owned by root:root, with mode 0o777.
It is a symlink to /usr/include/python2.7/../../lib/python2.7/dist-packages/mpi4py/include/mpi4py
/usr/include/python2.7/../../lib/python2.7/dist-packages/mpi4py/include/mpi4py