Remove six

Replace the following items with Python 3 style code.

- six.string_types
- six.integer_types
- six.moves
- six.PY2

Implements: blueprint six-removal

Change-Id: I2a0624bd4b455c7e5a0617f1253efa05485dc673

doc/source/contributor/plugin/strategy-plugin.rst

@@ -56,9 +56,6 @@ Here is an example showing how you can write a plugin called ``NewStrategy``:
     # filepath: thirdparty/new.py
     # import path: thirdparty.new
     import abc
-
-    import six
-
     from watcher._i18n import _
     from watcher.decision_engine.strategy.strategies import base