Remove uses of `six` from test_env.py.

https://crrev.com/c/2229335 added a dependency on `six` from
test_env.py in order to try and make it compatible with both
Python 2 and Python 3. However, it got the path to the
//third_party version of six wrong, and so it actually only worked
if six was available otherwise in the Python install (which it is,
most of the time, these days).

The script didn't actually benefit much from six, though, so
rather than fix the import path, this CL just makes the code
2- and 3-compatible without needing six, and removes the dependency
instead.

Bug: 942720
Change-Id: I44e44772cea8d92eff10a5c59879ce4d05de8fcc
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/2545924
Commit-Queue: Dirk Pranke <dpranke@google.com>
Reviewed-by: Ben Pastene <bpastene@chromium.org>
Cr-Commit-Position: refs/heads/master@{#828587}

testing/test_env.py

@@ -13,15 +13,6 @@ import subprocess
 import sys
 import time
 
-SIX_DIR = os.path.join(
-    os.path.dirname(os.path.dirname(os.path.abspath(__file__))), 'third_party',
-    'six')
-sys.path.insert(0, SIX_DIR)
-
-try:
-  import six
-except ImportError:
-  raise Exception('Failed to import six. Run under vpython or install six.')
 
 # This is hardcoded to be src/ relative to this script.
 ROOT_DIR = os.path.dirname(os.path.dirname(os.path.abspath(__file__)))
@@ -354,8 +345,8 @@ def run_executable(cmd, env, stdoutfile=None):
 
   print('Additional test environment:\n%s\n'
         'Command: %s\n' % (
-        '\n'.join('    %s=%s' %
-            (k, v) for k, v in sorted(six.iteritems(env_to_print))),
+        '\n'.join('    %s=%s' % (k, v)
+                  for k, v in sorted(env_to_print.items())),
         ' '.join(cmd)))
   sys.stdout.flush()
   env.update(extra_env or {})