[autotest] Make cli/*.py Python3-compatible syntax-wise.

Note: This change intentionally making substantial changes
such as changing all string literals to byte literals.

BUG=chromium:990593
TEST=python3 -m python_compile cli/*.py

Change-Id: I27c6428e2271e8b24ac38dc17f3e76b06b435cba
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/third_party/autotest/+/2298217
Tested-by: Gregory Nisbet <gregorynisbet@google.com>
Reviewed-by: Mike Frysinger <vapier@chromium.org>
Commit-Queue: Gregory Nisbet <gregorynisbet@google.com>

cli/atest.py

diff --git a/cli/atest.py b/cli/atest.py
index 4a88424..524ad71 100644
--- a/cli/atest.py
+++ b/cli/atest.py
@@ -14,6 +14,8 @@
 execute() and output() methods.
 """
 
+from __future__ import print_function
+
 __author__ = 'jmeurin@google.com (Jean-Marc Eurin)'
 
 import os, sys, re, traceback
@@ -90,7 +92,7 @@
             results = action_obj.execute()
         except topic_common.CliError:
             pass
-        except Exception, err:
+        except Exception as err:
             traceback.print_exc()
             action_obj.generic_error("Unexpected exception: %s" % err)
         else: