[Buildbot] #3321: make compare_attrs type consistent

[Buildbot trac]

#3321: make compare_attrs type consistent
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Reporter:  ewong      |       Owner:  ewong
    Type:  undecided  |      Status:  assigned
Priority:  minor      |   Milestone:  undecided
 Version:  0.8.12     |  Resolution:
Keywords:             |
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Comment (by sa2ajj):

 As a side note: maybe we should just fail if it's a list?

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Ticket URL: <http://trac.buildbot.net/ticket/3321#comment:6>
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