test properties on a new-style class.

[pierreglaser]

Follow up on #329
Properties only work on new-style classes, that's why test_property is failing on Python2 tests.

[ogrisel]

Could you please add a comment to explain this?

[ogrisel]

BTW we should consider making a new major cloudpickle release that stop supporting Python 2 in the coming months :)

[ogrisel]

+1 for merge once the comment is added.

[codecov]

Codecov Report

Merging #332 into master will increase coverage by 12.44%.
The diff coverage is n/a.

@@             Coverage Diff             @@
##           master     #332       +/-   ##
===========================================
+ Coverage   53.05%   65.49%   +12.44%     
===========================================
  Files           2        2               
  Lines         852      855        +3     
  Branches      176      177        +1     
===========================================
+ Hits          452      560      +108     
+ Misses        365      269       -96     
+ Partials       35       26        -9

Impacted Files	Coverage Δ
cloudpickle/cloudpickle.py	89.6% <0%> (+16.93%)	⬆️

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