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'

slapos/format.py

@@ -150,9 +150,12 @@ def getPublicIPv4Address():
       return ipv4
 
 def callAndRead(argument_list, raise_on_error=True):
-  popen = subprocess.Popen(argument_list,
+  popen = subprocess.Popen(
+    argument_list,
     stdout=subprocess.PIPE,
-                           stderr=subprocess.STDOUT)
+    stderr=subprocess.STDOUT,
+    universal_newlines=True,
+  )
   result = popen.communicate()[0]
   if raise_on_error and popen.returncode != 0:
     raise ValueError('Issue while invoking %r, result was:\n%s' % (

slapos/tests/test_slapformat.py

@@ -950,5 +950,9 @@ class TestFormatConfig(SlapformatMixin):
 
   # TODO add more tests with config file
 
-if __name__ == '__main__':
-  unittest.main()
+
+class TestCallAndRead(unittest.TestCase):
+  def test_callAndRead(self):
+    self.assertEqual(
+      slapos.format.callAndRead(['echo', 'hello']),
+      (0, 'hello\n'))