Install requires should include 'six'

[peterbe]

I just wanted to give it a quick test in a fresh new 3.6 virtualenv:

▶ pip install pypeln
Collecting pypeln
  Downloading https://files.pythonhosted.org/packages/e4/e7/49cffe147b72ebcf4dd7964f409c5b225f2173265b0c12fa9e3fe295d956/pypeln-0.1.6.tar.gz
Building wheels for collected packages: pypeln
  Running setup.py bdist_wheel for pypeln ... done
  Stored in directory: /Users/peterbe/Library/Caches/pip/wheels/08/f7/35/53e47573a9e6893ceca469c0a8115cee5b33672cefe12c50bd
Successfully built pypeln
Installing collected packages: pypeln
Successfully installed pypeln-0.1.6

▶ python dummy.py
Traceback (most recent call last):
  File "dummy.py", line 1, in <module>
    from pypeln import thread as th
  File "/private/tmp/venv/lib/python3.6/site-packages/pypeln/__init__.py", line 10, in <module>
    from . import thread
  File "/private/tmp/venv/lib/python3.6/site-packages/pypeln/thread.py", line 143, in <module>
    from six.moves.queue import Queue, Empty, Full
ModuleNotFoundError: No module named 'six'

[cgarciae]

@peterbe Hey, thanks for reporting! I'll include it in the next release!

[cgarciae]

Change made in develop branch.