BLD: fix cythonized msgpack extension in setup.py GH5831
closes #5831

@yarikoptic, can you confirm this fixes the problem?

looks completely logical and seems to help! ;)  I will do proper rebuild across to see if no side-effects (although once again -- logical, so I am not expecting any ;) )
 no word from @yarikoptic, tested well for me. 

I'm going to pull the trigger on this one. What could possibly happen??
 as I said -- it was so logical ;-)  I think this aspect indeed was resolved for me as well
 no worries.
