obsdiscovery: adopt to calling convention change
Upstream commits 71d83b315778 and abce899c985f changed the calling
convention of setdiscovery._takefullsample().
We inspect the signature of the function before calling it so
we can pass the proper arguments.
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Copyright 2011 Peter Arrenbrecht <peter.arrenbrecht@gmail.com>
Logilab SA <contact@logilab.fr>
Pierre-Yves David <pierre-yves.david@ens-lyon.org>
Patrick Mezard <patrick@mezard.eu>
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