Dependency 'six' is not specified in setup.py

[luator]

mjcf2urdf depends on six but it is not listed requirement in setup.py.

Steps to reproduce:

1. Install with pipx (this uses an isolated venv): pipx install mjcf2urdf.
2. Run: mjcf2urdf results in

    Traceback (most recent call last):
      File "/home/felixwidmaier/.local/bin/mjcf2urdf", line 5, in <module>
        from mjcf2urdf.apps.mjcf2urdf import main
      File "/home/felixwidmaier/.local/pipx/venvs/mjcf2urdf/lib/python3.10/site-packages/mjcf2urdf/__init__.py", line 9, in <module>
        from mjcf2urdf.makedirs import makedirs
      File "/home/felixwidmaier/.local/pipx/venvs/mjcf2urdf/lib/python3.10/site-packages/mjcf2urdf/makedirs.py", line 2, in <module>
        import six
    ModuleNotFoundError: No module named 'six'