the libraries in thrift/lib/py depend on six

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

build/fbcode_builder/manifests/fbthrift

@@ -18,8 +18,9 @@ wangle
 rsocket-cpp
 fizz
 fmt
-libsodium
 googletest
+libsodium
+python-six
 zstd
 
 [cmake.defines.test=on]