spack_json: _strify should be a no-op in Python 3
spack_json: _strify should be a no-op in Python 3

_strify traverses *all* of the read-in data and converts strings, but
only for Python 2.  Ignore it in Python 3.

LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack,LLNL/spack