Add ConnectionError exception

Declare ConnectionRefused as subclass of ConnectionError
ConnectionRefused is actually special case of ConnectionError

Partial-Bug: #1396570

Change-Id: Ibb0189da5c88498569ea90eb85efd9be493c197a
Remove requirements-py3

All the requirements are now Python 3 compatible (or at least
installable) so let's remove our special case requirements-py3.txt and
test-requirements-py3.txt

Change-Id: I63b92deda05809842a04f42f5c03f7b718c2089b
Merge "Remove requirements-py3"
Merge "Add ConnectionError exception"