Hexahedral mesh elements for an L Bracket?

I try to import a hex mesh into fenics using the method as suggested in:
https://fenicsproject.discourse.group/t/transitioning-from-mesh-xml-to-mesh-xdmf-from-dolfin-convert-to-meshio/412/3?u=reinhardt

This did not work yet.

*** Error:   Unable to order quadrilateral cell.
*** Reason:  Cell is not orderable.
*** Where:   This error was encountered inside QuadrilateralCell.cpp.
*** Process: 0
*** 
*** DOLFIN version: 2019.1.0
*** Git changeset:  74d7efe1e84d65e9433fd96c50f1d278fa3e3f3f

But if I pull the dolfinx docker and say “import dolfinx”, I still get the error message

ModuleNotFoundError: No module named 'dolfinx'

What would currently be the best way to work with a hex mesh?