After merging DM-15098 it was found that sphgeom::HtmPixelization::pixel causes a segfault when called on Linux.
Some investigation has shown that this bug was prexisting (and not unit tested) and caused by an upstream pybind11 bug (https://github.com/pybind/pybind11/issues/1132).
Until that is fixed, work around it by changing the holder type of all sphgeom::Pixelization subclasses to std::unique_ptr<T>.