Fix git_cl -r compatibility issue with Python 3

iterkeys is no longer a thing in Python 3

Change-Id: I4d88d2a486db1e552ecac27f123219ac1e03bcaf
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2276208
Commit-Queue: Carlos Caballero <carlscab@google.com>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

git_cl.py

@@ -3605,7 +3605,7 @@ def CMDissue(parser, args):
       if issue:
         issue_branch_map.setdefault(int(issue), []).append(branch)
     if not args:
-      args = sorted(issue_branch_map.iterkeys())
+      args = sorted(issue_branch_map.keys())
     result = {}
     for issue in args:
       try: