Use git repo data (latest tag, current commit hash, etc) for building
a version number according PEP 440.

Features:

* Does not require to change source code of the project
* Tag-, file-, and callback-based versioning schemas are supported
* Templates for tag, dev and dirty versions are separated
* Templates support a lot of substitutions including git and
  environment information
* Well-documented

NOTE: Will need to have PYTHONPATH set to use as slackware's tomli
      package is too old and may cause problems.  It will need to see
      python3-tomli-opt.  (only needed for python <3.11)

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