iotbx/pdb/tst_pdb.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/iotbx/pdb/tst_pdb.py", line 811, in <module>
        run()
      File "/__w/1/modules/cctbx_project/iotbx/pdb/tst_pdb.py", line 787, in run
        exercise_combine_unique_pdb_files()
      File "/__w/1/modules/cctbx_project/iotbx/pdb/tst_pdb.py", line 280, in exercise_combine_unique_pdb_files
        """)
    AssertionError