clean up usage of the six module

Now that we only support Python 3, we can drop all six usage.

BUG=b:187789302
TEST=CQ passes

Change-Id: I92ec7022f1f92fccc7d82ef3f262557081c0fc1b
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/chromite/+/2910860
Tested-by: Mike Frysinger <vapier@chromium.org>
Auto-Submit: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Chris McDonald <cjmcdonald@chromium.org>
Reviewed-by: Chris McDonald <cjmcdonald@chromium.org>
diff --git a/scripts/upload_symbols_unittest.py b/scripts/upload_symbols_unittest.py
index 37e0a7a..93bc0a3 100644
--- a/scripts/upload_symbols_unittest.py
+++ b/scripts/upload_symbols_unittest.py
@@ -5,17 +5,17 @@
 """Unittests for upload_symbols.py"""
 
 import errno
+import http.server
 import itertools
 import os
 import signal
 import socket
+import socketserver
 import sys
 import time
+import urllib.request
 
 import pytest  # pylint: disable=import-error
-from six.moves import BaseHTTPServer
-from six.moves import socketserver
-from six.moves import urllib
 
 from chromite.third_party import mock
 
@@ -117,7 +117,7 @@
     return result
 
 
-class SymbolServerRequestHandler(BaseHTTPServer.BaseHTTPRequestHandler):
+class SymbolServerRequestHandler(http.server.BaseHTTPRequestHandler):
   """HTTP handler for symbol POSTs"""
 
   RESP_CODE = None
@@ -138,7 +138,7 @@
     """Stub the logger as it writes to stderr"""
 
 
-class SymbolServer(socketserver.ThreadingTCPServer, BaseHTTPServer.HTTPServer):
+class SymbolServer(socketserver.ThreadingTCPServer, http.server.HTTPServer):
   """Simple HTTP server that forks each request"""