leverage six package for version compatibility

[laroque]

The six package is specifically made for compatibility between python 2 and 3 (2*3 = 6). We use should make use of this for as long as we're still using/supporting python 2. Of particular use, six.string_types is the right check when testing if something is a string (python 2 and 3 have rather different notions about what it means to be a "string" or "unicode" object).