Fix compatibility with setuptools 58 (or use an older version)

The latest version of the container used for deployment contains setuptools 59.6.0 (from Python 3.6.15), but since 58.0.0 it does not allow use_2to3 anymore. See https://setuptools.pypa.io/en/latest/history.html#v58-0-0

We could use an older version of Python (3.6.13 used to work) or move to a unified code base.