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/git_migrate_default_branch.py b/git_migrate_default_branch.py
index 4e00d3d..11ff4b6 100644
--- a/git_migrate_default_branch.py
+++ b/git_migrate_default_branch.py
@@ -11,7 +11,7 @@
 import scm
 import sys
 import logging
-from six.moves import urllib
+import urllib.parse
 
 
 def GetGerritProject(remote_url):