Reland "Drop py2 support in gerrit and git related files"

This is a reland of commit b5c7f4b46c3309d46e1796bce7fa83388af9bfd8

Original change's description:
> Drop py2 support in gerrit and git related files
>
> python3 is the only supported version of python in depot_tools.
>
> Bug: 1475402
> Change-Id: Ie4ee18d297081b3aa0206b8d7ce6461819bff0ca
> Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4809560
> Reviewed-by: Josip Sokcevic <sokcevic@chromium.org>
> Commit-Queue: Gavin Mak <gavinmak@google.com>

Bug: 1475402
Change-Id: I194180494071777b7b9dd91a5c8edabbbf5484c1
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/4811218
Reviewed-by: Joanna Wang <jojwang@chromium.org>
Commit-Queue: Gavin Mak <gavinmak@google.com>
diff --git a/gerrit_util.py b/gerrit_util.py
index 8312bba..1aeb94c 100644
--- a/gerrit_util.py
+++ b/gerrit_util.py
@@ -8,9 +8,6 @@
 https://gerrit-review.googlesource.com/Documentation/rest-api.html
 """
 
-from __future__ import print_function
-from __future__ import unicode_literals
-
 import base64
 import contextlib
 import httplib2
@@ -33,15 +30,10 @@
 import metrics_utils
 import subprocess2
 
-from third_party import six
 from six.moves import urllib
 
-if sys.version_info.major == 2:
-  import cookielib
-  from StringIO import StringIO
-else:
-  import http.cookiejar as cookielib
-  from io import StringIO
+import http.cookiejar
+from io import StringIO
 
 LOGGER = logging.getLogger()
 # With a starting sleep time of 12.0 seconds, x <= [1.8-2.2]x backoff, and six
@@ -255,7 +247,7 @@
 
   def _get_auth_for_host(self, host):
     for domain, creds in self.gitcookies.items():
-      if cookielib.domain_match(host, domain):
+      if http.cookiejar.domain_match(host, domain):
         return (creds[0], None, creds[1])
     return self.netrc.authenticators(host)