remove more py2 relicts

some u"" (double quotes) were not found in change before

Change-Id: I85912892bbe7b57afee758c69d1aa7073557adc0
Reviewed-on: https://forge.frm2.tum.de/review/c/sine2020/secop/playground/+/21326
Tested-by: JenkinsCodeReview <bjoern_pedersen@frm2.tum.de>
Reviewed-by: Bjoern Pedersen <bjoern.pedersen@frm2.tum.de>
Reviewed-by: Markus Zolliker <markus.zolliker@psi.ch>

secop/params.py

@@ -232,7 +232,7 @@ class Override(CountedObj):
                 return type(obj)(ctr=self.ctr, **props)
             return type(obj)(**props)
         raise ProgrammingError(
-            u"Overrides can only be applied to Accessibles, %r is none!" %
+            "Overrides can only be applied to Accessibles, %r is none!" %
             obj)