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/mainwindow.py

@@ -136,7 +136,7 @@ class MainWindow(QMainWindow):
             reply = self.show_save_message(self.tabWidget.tabText(index))
         if reply == QMessageBox.Cancel:
             return
-        elif reply == QMessageBox.Save:
+        if reply == QMessageBox.Save:
             self.save_tab(index)
         self.tabWidget.removeTab(index)
 
@@ -158,7 +158,7 @@ class MainWindow(QMainWindow):
             if reply == QMessageBox.Cancel:
                 event.ignore()
                 return
-            elif reply == QMessageBox.Save:
+            if reply == QMessageBox.Save:
                 for i in range(0, self.tabWidget.count()):
                     self.save_tab(i)
         event.accept()