ENH: Add dependency to "six" project to facilitate support of both py…

…2 and py3

Six provides simple utilities for wrapping over differences between
Python 2 and Python 3. It is intended to support codebases that work
on both Python 2 and 3 without modification. six consists of only one
Python file, so it is painless to copy into a project.

See https://pythonhosted.org/six/

requirements.txt

@@ -5,4 +5,5 @@ nose-parameterized>=0.5.0
 tqdm>=4.7.1
 PyWavelets>=0.4.0
 pykwalify>=1.6.0
+six>=1.10.0
 sphinx>=1.4