ENH: core: add hack enabling unpickling Py2 pickled scalars on Py3 under encoding='latin1'

There is a similar hack in place for arrays, but scalar unpickling was not covered.

Provides a workaround for gh-4879
Merge pull request #4883 from pv/fix-scalar-unpickle-py3

ENH: core: add hack enabling unpickling Py2 pickled scalars on Py3 under encoding='latin1'