math/py-scikit-umfpack: Change USE_PYTHON=cython3

Change USE_PYTHON=cython3 to USE_PYTHON=cython.

See PR 291778

While here, change /usr/local to ${LOCALBASE}

PR: 295541
Reviewed by: thierry (mentor), yuri (maintainer)
Approved by: thierry (mentor), yuri (maintainer)