tests failure with pypy3

[thiell]

Low priority, but good to know and if ever someone is interested to fix it, here is the only issue:

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ERROR: Failure: ImportError (No module named 'cPickle')
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Traceback (most recent call last):
  File "/home/travis/build/thiell/clustershell/lib/ClusterShell/Communication.py", line 46, in <module>
    import _pickle as cPickle
ImportError: No module named '_pickle'

$ python --version
Python 3.5.3 (a39af0be3a22, Jun 05 2017, 20:18:00)
[PyPy 5.8.0-beta0 with GCC 6.2.0 20160901]
$ pip --version
pip 9.0.1 from /home/travis/virtualenv/pypy3.5-5.8.0/site-packages (python 3.5)