format: fix callAndRead for python3

Fixes the following error:

Traceback (most recent call last):
  File "/opt/slapos/eggs/slapos.core-1.7.9-py3.7.egg/slapos/cli/entry.py", line 305, in run_subcommand
    result = cmd.run(parsed_args)
  File "/opt/slapos/eggs/slapos.core-1.7.9-py3.7.egg/slapos/cli/command.py", line 56, in run
    return self.take_action(parsed_args)
  File "/opt/slapos/eggs/slapos.core-1.7.9-py3.7.egg/slapos/cli/format.py", line 128, in take_action
    do_format(conf=conf)
  File "/opt/slapos/eggs/slapos.core-1.7.9-py3.7.egg/slapos/format.py", line 1396, in do_format
    create_tap=conf.create_tap)
  File "/opt/slapos/eggs/slapos.core-1.7.9-py3.7.egg/slapos/format.py", line 655, in format
    partition.address_list.append(self.interface.addIPv6Address())
  File "/opt/slapos/eggs/slapos.core-1.7.9-py3.7.egg/slapos/format.py", line 1223, in addIPv6Address
    if self._addSystemAddress(addr, netmask, tap=tap):
  File "/opt/slapos/eggs/slapos.core-1.7.9-py3.7.egg/slapos/format.py", line 1086, in _addSystemAddress
    for l in result.split('\n'):
TypeError: a bytes-like object is required, not 'str'