remove six usage

Summary:
X-link: facebookincubator/zstrong#1002

now that python2 is long gone, lets remove dependency on six

most of the changes are in the thrift fuzzer/randomizer used in thrift tests, replacing six with what six did in the py3 case.

Reviewed By: ahilger

Differential Revision: D63709297

fbshipit-source-id: 2ff46ead9e2035c5e43081ddec8ae1e3d1499895

build/fbcode_builder/manifests/fbthrift

@@ -23,7 +23,6 @@ fmt
 folly
 googletest
 libsodium
-python-six
 wangle
 zstd
 mvfst