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/miniplot.py

@@ -44,7 +44,7 @@ _orange = QBrush(QColor('#ffa500'))
 
 my_uipath = path.dirname(__file__)
 
-class MiniPlotCurve(object):
+class MiniPlotCurve:
     # placeholder for data
     linecolor = _black
     linewidth = 0  # set to 0 to disable lines