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_mlz/amagnet.py

@@ -116,7 +116,7 @@ class GarfieldMagnet(SequencerMixin, Drivable):
             if field == tryfield:
                 self.log.debug('current for %g T is %g A', field, trycurr)
                 return trycurr  # Gotcha!
-            elif field > tryfield:
+            if field > tryfield:
                 # retry upper interval
                 mincurr = trycurr
                 minfield = tryfield