pksetup.pkdeploy fails with SyntaxError in itertools

[robnagler]

Caused by pypa/setuptools#2687

The error is:

raise type(self._exception), self._exception, self._traceback
                                     ^
      SyntaxError: invalid syntax

This only happens because we are using tox, which does a clean install to build an sdist. I think we need to dump tox to build sdists. But documenting this here.