further fixes of py3 issues

complaints by pylint are mainly related to
- remove object from base list in class definitions
- unnecessary else/elif after return/raise

Change-Id: I13d15449149cc8bba0562338d0c9c42e97163bdf
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/21325
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/gui/cfg_editor/node_display.py

@@ -28,7 +28,7 @@ class NodeDisplay(QWidget):
     def __init__(self, file_path=None, parent=None):
         QWidget.__init__(self, parent)
         loadUi(self, 'node_display.ui')
-        self.saved = True if file_path else False
+        self.saved = bool(file_path)
         self.created = self.tree_widget.set_file(file_path)
         self.tree_widget.save_status_changed.connect(self.change_save_status)
         self.tree_widget.currentItemChanged.connect(self.set_scroll_area)