Faulty nogil assumption in _Py_HashDouble
Pandas uses _Py_HashDouble as the hash function for hashing floats. () in the 0.17.x and 0.18.x releases. This hash function is used within nogil sections within hashtable.pyx on both these releases. However the definition of this function as implemented on Python 2.x can call into the Python api depending on the values being hashed. This causes crashes if there is another thread using holding the GIL which is executing simultaneously. I see that master no longer uses this hash function. But can this fix be backported for the 0.17.x / 0.18.x branches? 
