Use quote/unquote from six module for py3

Change-Id: I42cff32fbf3febd6dd6a7171b9214f93b4414f9a
Remove validate_headers

It wasn't used anymore since moved up to requests (and it fails in py3).

Change-Id: Ic8a80ae09ca6445696a9cf34ffb503c5ff51bc79
Merge "Use quote/unquote from six module for py3"
Merge "Remove validate_headers"