Fix python2.6 SyntaxError.

[Dr15Jones]

FWCore/ParameterSet/python/Mixins.py uses syntax that is not valid
in python2.6; breaks ability to import this file in current CMSSW_7_6_X
(and hence submit to CRAB).

Backported from CMSSW_7_6.

[cmsbuild]

A new Pull Request was created by @Dr15Jones (Chris Jones) for CMSSW_7_5_X.

Fix python2.6 SyntaxError.

It involves the following packages:

FWCore/ParameterSet

@cmsbuild, @smuzaffar, @Dr15Jones can you please review it and eventually sign? Thanks.
@Martin-Grunewald, @wddgit, @wmtan this is something you requested to watch as well.
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@Degano you are the release manager for this.
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[Dr15Jones]

please test

[Dr15Jones]

+1

[cmsbuild]

The tests are being triggered in jenkins.
https://cmssdt.cern.ch/jenkins/job/ib-any-integration/8756/console

[cmsbuild]

This pull request is fully signed and it will be integrated in one of the next CMSSW_7_5_X IBs after it passes the integration tests and once validation in the development release cycle CMSSW_7_6_X is complete. This pull request requires discussion in the ORP meeting before it's merged. @davidlange6, @Degano, @smuzaffar

[cmsbuild]

+1
Tested at: e8ec43e
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-11724/8756/summary.html

[cmsbuild]

Comparison is ready
https://cmssdt.cern.ch/SDT/jenkins-artifacts/pull-request-integration/PR-11724/8756/summary.html