depot_tools: bugfix for py3 syntax and bytes/str changes

Also update copywrite header to conform to expectations.

BUG=chromium:1082354
TEST=locally, can upload commit with this fix

Change-Id: I5527cf9cc58fbab94f777b82008a422a3190f539
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/tools/depot_tools/+/2199884
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Edward Lesmes <ehmaldonado@chromium.org>

presubmit_canned_checks.py

@@ -1513,7 +1513,7 @@ def CheckChangedLUCIConfigs(input_api, output_api):
     req.add_header('Authorization', 'Bearer %s' % acc_tkn.token)
     if body is not None:
       req.add_header('Content-Type', 'application/json')
-      req.add_data(json.dumps(body))
+      req.data = json.dumps(body).encode('utf-8')
     return json.load(input_api.urllib_request.urlopen(req))
 
   try: