Salt 'debconf' states fail since move to Python 3

Since the move to salt running under Python 3, salt 'debconf' states fail with:

```
[ERROR ] An exception occurred in this state: Traceback (most recent call last):
  File "/usr/lib/python3/dist-packages/salt/state.py", line 1851, in call
    **cdata['kwargs'])
  File "/usr/lib/python3/dist-packages/salt/loader.py", line 1795, in wrapper
    return f(*args, **kwargs)
  File "/usr/lib/python3/dist-packages/salt/states/debconfmod.py", line 197, in set
    if __salt__['debconf.set'](name, key, args['type'], args['value']):
  File "/usr/lib/python3/dist-packages/salt/modules/debconfmod.py", line 130, in set_
    os.write(fd_, line)
TypeError: a bytes-like object is required, not 'str'
```

ProblemType: Bug
DistroRelease: Ubuntu 18.04
Package: salt-minion 2017.7.3+dfsg1-1
ProcVersionSignature: Ubuntu 4.15.0-10.11-generic 4.15.3
Uname: Linux 4.15.0-10-generic x86_64
ApportVersion: 2.20.8-0ubuntu10
Architecture: amd64
Date: Thu Mar 1 16:42:41 2018
JournalErrors:
 -- Logs begin at Mon 2018-01-29 14:53:26 GMT, end at Thu 2018-03-01 16:42:31 GMT. --
 Mar 01 16:38:24 hostname kernel: piix4_smbus 0000:00:07.3: SMBus Host Controller not enabled!
 Mar 01 16:38:24 hostname kernel: sd 2:0:0:0: [sda] Assuming drive cache: write through
PackageArchitecture: all
ProcEnviron:
 LANGUAGE=en_GB:
 TERM=xterm-256color
 PATH=(custom, no user)
 LANG=en_GB.UTF-8
 SHELL=/bin/bash
SourcePackage: salt
UpgradeStatus: No upgrade log present (probably fresh install)