Fix hstore_plpython for Python 3.

In commit d51924be, I overlooked the need to provide linkage for
PLyUnicode_FromStringAndSize, because that's only used (and indeed
only exists) in Python 3 builds.

In light of the need to #if this item, rearrange the ordering of
the code related to each function pointer, so as not to need more
#if's than absolutely necessary.

Per buildfarm.