Summary:
The libraries in thrift/lib/py support both Python 2 and Python 3, and rely on
the Python six module for some of this compatibility support.

Update the getdeps manifest for fbthrift to indicate this dependency, and
update fbthrift's CMakeLists.txt file to find and reference python-six
properly.  This will ensure that the python-six code is built into any python
executable that uses the thrift/lib/py libraries.

Reviewed By: yfeldblum

Differential Revision: D17401218

fbshipit-source-id: 0007dda8974ae9bd87e4d7e256c74908c9a30d8f