Ronald,

I thought it could be an `atomic` issue by the fact that test_rename transforms a filename in tmp and then it change again to the original name. Not being atomic, the rename will not finish the executation of the first one (even though a return code has been sent) and FileNotFoundError is dispatched. 

Anyway, i've added a time.sleep in the middle of the two lines to assert this and no more intermittence could be found. Ideas?