Mango python test suite doesn't run under Python 3

[wohali]

https://builds.apache.org/blue/organizations/jenkins/CouchDB/detail/master/82/pipeline

Our CentOS 6 build runs under Python 3, because it's annoying to get an up-to-date Python 2 on CentOS 6 but not annoying to get a Python 3.

There are a number of syntax errors that need to be resolved pronto. master will continue to fail on Jenkins until they do.

@tonysun83 @garrensmith can you take a look here? The error log for the centos 6 build is pretty explicit as to where the errors are. 2to3 will also highlight these for you.

[tonysun83]

@wohali: I'm on it.

[tonysun83]

PR : #839