Add pyproject.toml file

[JAlvarezJarreta]

This eliminates the need to populate $PYTHONPATH by running pip install .. Furthermore, to use these runnables in eHive, it is as easy as doing:

    {
      -logic_name => 'analysis_name',
      -module     => 'bio.ensembl.ontology.hive.OLSHiveLoader',
      -language   => 'python3',
      ...

[JAlvarezJarreta]

@dpopleton shall I close this PR?