depot_toools: Make subcommand Python 3 compatible.

Bug: 984182
Change-Id: I5d1b5626eb799975539560ca1508af92df326d47
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/1721916
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Auto-Submit: Edward Lesmes <ehmaldonado@chromium.org>

subcommand.py

@@ -160,7 +160,7 @@ class CommandDispatcher(object):
     commands = self.enumerate_commands()
     docs = sorted(
         (cmd_name, self._create_command_summary(cmd_name, handler))
-        for cmd_name, handler in commands.iteritems())
+        for cmd_name, handler in commands.items())
     # Skip commands without a docstring.
     docs = [i for i in docs if i[1]]
     # Then calculate maximum length for alignment: