BUG: Prevent 3D-ndarray for nested tuple labels (#24687)
Being a very rare issue encountered with nested tuples as column and index labels, here is the fix I've managed to come up with - for the time being.  While `clean_index_list()` in `pandas/_libs/lib.pyx` is responsible for returning an invalid result (a 3D ndarray where the inner dimensions should be nested tuples), debugging Cython is very challenging for me.  And yes, use of tuples in this way is extremely uncommon.  So far, the code has run successfully on two distance matrices.
<img width="1191" alt="fix" src="https://user-images.githubusercontent.com/17303881/51058729-b9af9080-15b7-11e9-9726-3bde8ca67330.png">
