[Buildbot] #3321: make compare_attrs type consistent

[Buildbot trac]

#3321: make compare_attrs type consistent
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Reporter:  ewong      |      Owner:
    Type:  undecided  |     Status:  new
Priority:  major      |  Milestone:  undecided
 Version:  0.8.12     |   Keywords:
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 Currently, in 0.8.12 in some part of the code, compare_attrs is a tuple
 and in other parts, it's a list.

 For consistency's sake, perhaps it's better to make them the same
 type throughout the code?

 As Pierre mentioned on the list:
 {{{
   "Each class should declare the attributes they care about, and the
 ComparableMixin should gather and concatenate everything."
 }}}

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Ticket URL: <http://trac.buildbot.net/ticket/3321>
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