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/protocol/messages.py

@@ -80,7 +80,7 @@ REQUEST2REPLY = {
 
 
 
-HelpMessage = u"""Try one of the following:
+HelpMessage = """Try one of the following:
             '%s' to query protocol version
             '%s' to read the description
             '%s <module>[:<parameter>]' to request reading a value