Add `six` Python module to lldb/third_party.

Six is a python module designed to smooth the process of porting
Python 2 code to Python 3.  Specifically, six provides a consistent
interface to some of the breaking changes between 2 and 3.  For example,
the syntax for assigning a metaclass differs in Python 2 and 3.  Six
addresses this by providing a single class decorator that will do the
right thing depending on which version of Python is being run.

There are other examples too, such as dealing with renamed modules,
unicode literals, etc.

llvm-svn: 250857