setup.py: install_requires=['six>=1.11.0']

We will require the popular [__six__](https://pypi.org/project/six) module to simplify our support of both Python 2 and Python 3.

setup.py

@@ -60,7 +60,8 @@ setup(
                        '*.h', '*.cpp', '*.bash', '*.txt',
                        '*.dll', '*.lib', '*.so', '*.pyd',
                        '*.dat', '*.ini', '*.pfx', '*.scc', '*.crt', '*.key']},
+    install_requires=['six>=1.11.0'],
     extras_require={
         'tq': ["tornado>=4.5.1", "sortedcontainers>=1.5.7"],
     }
-)
+)