git-cl: Make tests run on Python 3.

Bug: 1002209
Change-Id: I90de660afd901e544e5557f3af3a56cef4d6adaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1891667
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Anthony Polito <apolito@google.com>
diff --git a/gerrit_util.py b/gerrit_util.py
index 62dae27..cf88e7d 100644
--- a/gerrit_util.py
+++ b/gerrit_util.py
@@ -917,7 +917,7 @@
   Invalid accounts, either not existing or without unique match,
   are not present as returned dictionary keys.
   """
-  assert not isinstance(accounts, basestring), type(accounts)
+  assert not isinstance(accounts, str), type(accounts)
   accounts = list(set(accounts))
   if not accounts:
     return {}