cctbx/xray/boost_python/tst_xray.py broken on Python 3

[Anthchirp]

loads of tests still failing on python 3. This one is run by azure:
https://dev.azure.com/cctbx/cctbx_project/_build/results?buildId=184

  Standard error:
    Traceback (most recent call last):
      File "/__w/1/modules/cctbx_project/cctbx/xray/boost_python/tst_xray.py", line 2010, in <module>
        run()
      File "/__w/1/modules/cctbx_project/cctbx/xray/boost_python/tst_xray.py", line 1996, in run
        exercise_scattering_type_registry()
      File "/__w/1/modules/cctbx_project/cctbx/xray/boost_python/tst_xray.py", line 894, in exercise_scattering_type_registry
        type_index_pairs, unique_gaussians, unique_counts)
    TypeError: Expecting an object of type list; got an object of type dict_keys instead