dot_to_qt.py: Another small fix for Python 3 compatibility

[kartikmohta]

No description provided.

[dirk-thomas]

Please provide a reproducible example.

[kartikmohta]

rqt_graph crashes in Python 3 when set to show "Nodes only", unchecking the "Hide: Debug" box and pressing the refresh button. Don't need any nodes running, just roscore.

Traceback (most recent call last):
  File "/opt/ros/lunar_py3/lib/python3.6/site-packages/rqt_graph/ros_graph.py", line 231, in _refresh_rosgraph
    self._update_graph_view(self._generate_dotcode())
  File "/opt/ros/lunar_py3/lib/python3.6/site-packages/rqt_graph/ros_graph.py", line 266, in _update_graph_view
    self._redraw_graph_view()
  File "/opt/ros/lunar_py3/lib/python3.6/site-packages/rqt_graph/ros_graph.py", line 299, in _redraw_graph_view
    same_label_siblings=True)
  File "/opt/ros/lunar_py3/lib/python3.6/site-packages/qt_dotgraph/dot_to_qt.py", line 249, in dotcode_to_qt_items
    edges = self.parse_edges(graph, nodes, highlight_level, same_label_siblings)
  File "/opt/ros/lunar_py3/lib/python3.6/site-packages/qt_dotgraph/dot_to_qt.py", line 298, in parse_edges
    same_label_siblings=same_label_siblings)
  File "/opt/ros/lunar_py3/lib/python3.6/site-packages/qt_dotgraph/dot_to_qt.py", line 193, in addEdgeItem
    label = label.decode('string_escape')
AttributeError: 'str' object has no attribute 'decode'

[dhood]

The decoding was added in 18a38c8 to unescape backslash-escaped newlines in labels, which continues to work with this PR (tested manually by appending '\\nά). The type of label changes from str to unicode but I don't think there are negative side effects in python2, so LGTM.