Fix missing six requirement

After a fresh install

$ python3 -m venv venv
$ . venv/bin/activate
$ pip3 install pywebpush

pywebpush fails with a missing dependency:

$ python3 -c "import pywebpush"
Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/tmp/pywebpush/pywebpush/__init__.py", line 16, in <module>
    import six
ModuleNotFoundError: No module named 'six'

six was never listed in requirements.txt, so my guess would be that so far it has been installed by an upstream dependency, which recently dropped it.

This small fix adds six to requirements.txt.

Thanks so much for your work on pywebpush! :blush:

web-push-libs / pywebpush

Fix missing six requirement #132