python3-hatchling (Hatch build backend)

This is the extensible, standards compliant build backend used by
Hatch.

Add the following before the python3 build commands in your
Slackbuild as hatchling now requires python3-packaging-opt:

PYVER=$(python3 -c 'import sys; print("%d.%d" % sys.version_info[:2])')
export PYTHONPATH=/opt/python$PYVER/site-packages

NOTE: As of version 1.28.0 hatchling has dropped support for python3.9.
      However, it still seems to work fine so I will continue updating
      it for the time being.