ModuleNotFoundError: No module named 'gunicorn.six' (python2 and python3)

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Hello, I am setting up the oncall to give it a try.

I switched to py3 branch, followed the docs and running into following error:

```
oncall-dev ./configs/config.yaml
Traceback (most recent call last):
File "/home/oncall/.virtualenvs/p3-oncall/bin/oncall-dev", line 11, in
load_entry_point('oncall', 'console_scripts', 'oncall-dev')()
File "/home/oncall/.virtualenvs/p3-oncall/lib/python3.6/site-packages/pkg_resources/__init__.py", line 480, in load_entry_point
return get_distribution(dist).load_entry_point(group, name)
File "/home/oncall/.virtualenvs/p3-oncall/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2693, in load_entry_point
return ep.load()
File "/home/oncall/.virtualenvs/p3-oncall/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2324, in load
return self.resolve()
File "/home/oncall/.virtualenvs/p3-oncall/lib/python3.6/site-packages/pkg_resources/__init__.py", line 2330, in resolve
module = __import__(self.module_name, fromlist=['__name__'], level=0)
File "/home/oncall/oncall/src/oncall/bin/run_server.py", line 10, in
from gunicorn.six import iteritems
ModuleNotFoundError: No module named 'gunicorn.six'
Makefile:4: recipe for target 'serve' failed
make: *** [serve] Error 1
```

Start by reproducing the failure with `oncall-dev ./configs/config.yaml` on the py3 branch, then inspect `src/oncall/bin/run_server.py` at the `gunicorn.six` import. Check the related dependency setup and run the `make serve` target. Done means the server starts without the reported `ModuleNotFoundError` on the documented Python versions.

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