conveyor submitter crashes on python3

[tbeerman]

Motivation

The submitter crashed when run with python3 with the following message. This doesn't happen in python2.

CRITICAL        Thread [0/73] :  Traceback (most recent call last):
  File "/usr/local/lib/python3.6/site-packages/rucio/daemons/conveyor/submitter.py", line 170, in submitter
    retry_other_fts=retry_other_fts)
  File "/usr/local/lib/python3.6/site-packages/rucio/daemons/conveyor/submitter.py", line 337, in __get_transfers
    failover_schemes=failover_schemes)
  File "/usr/local/lib/python3.6/site-packages/rucio/db/sqla/session.py", line 377, in new_funct
    result = function(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/rucio/core/transfer.py", line 723, in get_transfer_requests_and_source_replicas
    session=session)
  File "/usr/local/lib/python3.6/site-packages/rucio/db/sqla/session.py", line 393, in new_funct
    result = function(*args, **kwargs)
  File "/usr/local/lib/python3.6/site-packages/rucio/core/transfer.py", line 582, in get_hops
    scheme=limit_dest_schemes if out_v == dest_rse_id and limit_dest_schemes else None)
  File "/usr/local/lib/python3.6/site-packages/rucio/rse/rsemanager.py", line 712, in find_matching_scheme
    dest_candidates = sorted(dest_candidates, key=lambda k: k['domains'][domain][operation_dest])
TypeError: '<' not supported between instances of 'NoneType' and 'int'