Remove use of six library

It exists only for py2/py3 compat. We do not need it any more.

This will explicitly break Zuul v3 for python2, which is different than
simply ceasing to test it and no longer declaring we support it. Since
we're not testing it any longer, it's bound to degrade overtime without
us noticing, so hopefully a clean and explicit break will prevent people
from running under python2 and it working for a minute, then breaking
later.

Change-Id: Ia16bb399a2869ab37a183f3f2197275bb3acafee

zuul/driver/bubblewrap/__init__.py

@@ -19,11 +19,10 @@ import grp
 import logging
 import os
 import pwd
+import shlex
 import subprocess
 import sys
 
-from six.moves import shlex_quote
-
 from zuul.driver import (Driver, WrapperInterface)
 
 
@@ -144,7 +143,7 @@ class BubblewrapDriver(Driver, WrapperInterface):
         command = [x.format(**kwargs) for x in bwrap_command]
 
         self.log.debug("Bubblewrap command: %s",
-                       " ".join(shlex_quote(c) for c in command))
+                       " ".join(shlex.quote(c) for c in command))
 
         wrapped_popen = WrappedPopen(command, passwd_r, group_r)