Flake8: Four F821 undefined name issues

[cclauss]

flake8 testing of https://github.com/plotly/dash on Python 3.6.2

$ flake8 . --count --select=E901,E999,F821,F822,F823 --show-source --statistics

./setup.py:8:13: F821 undefined name '__version__'
    version=__version__,
            ^

./dash/dash.py:395:15: F821 undefined name 'DashException'
        raise DashException(
              ^

./dash/dash.py:423:43: F821 undefined name 'arg'
                        name.lower(), str(arg), name
                                          ^

./tests/test_authentication.py:310:13: F821 undefined name 'token'
            token
            ^