A simple, correct PEP517 package builder.

NOTE:
  Has a soft requirement for python-importlib_metadata (and python-zipp)
  which should be installed as it fixes a stdlib bug, but newer versions
  have a circular dep on this slackbuild so cannot be built before it.
  Therefore it will NOT be added to REQUIRES here.

  This change will not affect most python builds.

NOTE: newer versions have a requirement for "packaging >= 24.0".  This
      would require modifying every dependee script to use *-opt
      packages.  If a newer version is needed I will look at this
      again.
