Fix unpickling series with non unique index

Series, frames and panels that contain only one block can be unpickled
under the assumption that block items correspond to manager items 1-to-1
(as pointed out in #7329).

I still don't have neither of darwin/win/32bit platforms at hand, so I
cannot post those pickles.  Also, my linux has a fresher python than
indicated by filenames of existing pickle test data.  I did generate
some data to test the fix, the question is do you want me to upload it?