cctbx/regression/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/regression/tst_xray.py", line 1465, in <module>
        run()
      File "/__w/1/modules/cctbx_project/cctbx/regression/tst_xray.py", line 1458, in run
        exercise_structure()
      File "/__w/1/modules/cctbx_project/cctbx/regression/tst_xray.py", line 318, in exercise_structure
        l = pickle.loads(p)
    TypeError: Expecting an object of type list; got an object of type dict_keys instead

similar to #365