[Twisted-Python] Using six for Python3 porting

[Adi Roiban]

Hi,

I feel that twisted.python.compat is slowly duplicating / reinventing
an important part of the six code.

Code which does not import from  twisted.python.compat will have all
kind of ad-hoc / in module code to handle differences between py2 and
py3 and many if _PY3 conditional blocks.

What do you say if we add six as dependency for twisted ?

In case you don't want six as a dependency, it can be copied in
twisted code base and make it available from twisted.python.six or a
similar package.

-- 
Adi Roiban