Exception raised when opening showscan in Sardana spock with Python 3 and PyMCA 5.1.3 (default version on Debian 9). Fixed in PyMca 5.5.

pymca5.1.3-py3

```
Door_demo2_1 [25]: showscan
Trying to open local scan file /tmp/demo1.h5...
AttributeError: 'str' object has no attribute 'decode'
(For more detailed information type: python_error)

Door_demo2_1 [26]: python_error
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-25-47bb900506ec> in <module>()
----> 1 get_ipython().magic('showscan ')

/usr/lib/python3/dist-packages/IPython/core/interactiveshell.py in magic(self, arg_s)
   2156         magic_name, _, magic_arg_s = arg_s.partition(' ')
   2157         magic_name = magic_name.lstrip(prefilter.ESC_MAGIC)
-> 2158         return self.run_line_magic(magic_name, magic_arg_s)
   2159 
   2160     #-------------------------------------------------------------------------

/usr/lib/python3/dist-packages/IPython/core/interactiveshell.py in run_line_magic(self, magic_name, line)
   2077                 kwargs['local_ns'] = sys._getframe(stack_depth).f_locals
   2078             with self.builtin_trap:
-> 2079                 result = fn(*args,**kwargs)
   2080             return result
   2081 

<decorator-gen-129> in showscan(self, parameter_s)

/usr/lib/python3/dist-packages/IPython/core/magic.py in <lambda>(f, *a, **k)
    186     # but it's overkill for just that one bit of state.
    187     def magic_deco(arg):
--> 188         call = lambda f, *a, **k: f(*a, **k)
    189 
    190         if callable(arg):

/usr/local/lib/python3.5/dist-packages/sardana/spock/magic.py in showscan(self, parameter_s)
    127         else:
    128             scan_nb = int(params[0])
--> 129     door.show_scan(scan_nb, online=online)
    130 
    131 

/usr/local/lib/python3.5/dist-packages/sardana/spock/spockms.py in show_scan(self, scan_nb, online)
    435         self._plotter.show_scan(scan_nb=scan_nb,
    436                                 scan_history_info=scan_history_info,
--> 437                                 directory_map=directory_map)
    438 
    439     def stateChanged(self, s, t, v):

/usr/local/lib/python3.5/dist-packages/sardana/spock/spockms.py in show_scan(self, scan_nb, scan_history_info, directory_map)
    171 
    172         print("Trying to open local scan file %s..." % (local_file,))
--> 173         taurus_nexus_widget.openFile(local_file)
    174         taurus_nexus_widget.show()
    175         nexus_widget = taurus_nexus_widget.neXusWidget()

/usr/local/lib/python3.5/dist-packages/taurus/qt/qtgui/extra_nexus/taurusnexuswidget.py in openFile(self, fname)
    120             fname, _ = compat.getOpenFileName(self, 'Choose NeXus File', '')
    121         if fname:
--> 122             self.__nexusFile = self.__fileModel.openFile(fname)
    123 
    124     def onHDF5WidgetSignal(self, ddict):

/usr/lib/python3/dist-packages/PyMca5/PyMcaGui/io/hdf5/HDF5Widget.py in openFile(self, filename, weakreference)
    482             self.rootItem.appendChild(refProxy)
    483         else:
--> 484             self.rootItem.appendChild(phynxFile)
    485         ddict = {}
    486         ddict['event'] = "fileAppended"

/usr/lib/python3/dist-packages/PyMca5/PyMcaGui/io/hdf5/HDF5Widget.py in appendChild(self, item)
    165 
    166     def appendChild(self, item):
--> 167         self.children.append(H5FileProxy(item, self))
    168         self._identifiers.append(id(item))
    169 

/usr/lib/python3/dist-packages/PyMca5/PyMcaGui/io/hdf5/HDF5Widget.py in __init__(self, ffile, parent)
    332 
    333     def __init__(self, ffile, parent=None):
--> 334         super(H5FileProxy, self).__init__(ffile, ffile, parent)
    335         self._name = ffile.name
    336         self._filename = self.file.name

/usr/lib/python3/dist-packages/PyMca5/PyMcaGui/io/hdf5/HDF5Widget.py in __init__(self, ffile, node, parent, path)
    291                             _type = "%s" % node.attrs[cname]
    292                         else:
--> 293                             _type = node.attrs[cname].decode('utf=8')
    294                         self._type = _type
    295                         break

AttributeError: 'str' object has no attribute 'decode'
```