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>

test/test_basic_validators.py

@@ -29,7 +29,7 @@ from secop.basic_validators import FloatProperty, PositiveFloatProperty, \
     NonNegativeIntProperty, BoolProperty, StringProperty, UnitProperty, \
     FmtStrProperty, OneOfProperty, NoneOr, EnumProperty, TupleProperty
 
-class unprintable(object):
+class unprintable:
     def __str__(self):
         raise NotImplementedError