Include six as dependency

CHANGELOG.rst

@@ -4,7 +4,7 @@ pytest-xdist 1.22.5 (2018-07-27)
 Bug Fixes
 ---------
 
-- `#321 <https://github.com/pytest-dev/pytest-xdist/issues/321>`_: Revert change that dropped support for ``pytest<3.4``.
+- `#321 <https://github.com/pytest-dev/pytest-xdist/issues/321>`_: Revert change that dropped support for ``pytest<3.4`` and require ``six``.
 
   This change caused problems in some installations, and was a mistaken
   in the first place as we should not change version requirements

setup.py

@@ -1,6 +1,6 @@
 from setuptools import setup, find_packages
 
-install_requires = ['execnet>=1.1', 'pytest>=3.0.0', 'pytest-forked']
+install_requires = ['execnet>=1.1', 'pytest>=3.0.0', 'pytest-forked', "six"]
 
 
 setup(