Package six is missing after installation

[chbndrhnns]

Installing the package with pip install plantuml and afterwards running it with python -m plantuml fails due to the missing six package which needs to be installed manually:

python -m plantuml
\Traceback (most recent call last):
  File "/usr/local/lib/python3.6/runpy.py", line 193, in _run_module_as_main
    "__main__", mod_spec)
  File "/usr/local/lib/python3.6/runpy.py", line 85, in _run_code
    exec(code, run_globals)
  File "/usr/local/lib/python3.6/site-packages/plantuml.py", line 13, in <module>
    import six
ModuleNotFoundError: No module named 'six'

[jacebrowning]

I think 'six' needs to be added to install_requires.