title: rqt_graph pyqt binding of qt_gui_cpp library
body: <p>I am using ROS-melodic in ubuntu 18.04 LTS. I am getting this error:</p>
  <pre><code>Could not import &quot;pyqt&quot; bindings of qt_gui_cpp library - so C++ plugins will not be available:
  Traceback (most recent call last):
    File &quot;/opt/ros/melodic/lib/python2.7/dist-packages/qt_gui_cpp/cpp_binding_helper.py&quot;, line 43, in &lt;module&gt;
      from . import libqt_gui_cpp_sip
  ImportError: dynamic module does not define module export function (PyInit_libqt_gui_cpp_sip)
  </code></pre>
  <p>however I have installed pyqt5 and pydot manually, rqt_graph shows up but with this warning. I want to know what can I do to get it right? Is this going to be problem in future? Now rqt_graph is loading but I don't know if this will mess up/create lacking in the graph?</p>