python3.5 subprocess.check_output() doesn't support encoding argument

The command 'juju deploy cs:graylog-39 --config channel=3/stable --series xenial graylog2' will produce the following errors.

File "/var/lib/juju/agents/unit-graylog2-0/charm/reactive/graylog.py", line 95, in refresh_graylog
cur_snap = new_snap = snap.get_installed_version(SNAP_NAME)
File "lib/charms/layer/snap.py", line 256, in get_installed_version
return subprocess.check_output(cmd, encoding='utf-8').partition(
File "/usr/lib/python3.5/subprocess.py", line 626, in check_output
**kwargs).stdout
File "/usr/lib/python3.5/subprocess.py", line 693, in run
with Popen(*popenargs, **kwargs) as process:
TypeError: __init__() got an unexpected keyword argument 'encoding'

but the command 'juju deploy cs:graylog-39 --config channel=3/stable --series bionic graylog' won't.

xenial uses python3.5 as default, while bionic uses python3.6 as default.
and I found the function check_output doesn't have the encoding argument in python3.5 according to the doc [1].

So after changing L256 of /var/lib/juju/agents/unit-graylog2-0/charm/lib/charms/layer/snap.py, graylog charm run well again.

from

return subprocess.check_output(cmd, encoding='utf-8').partition(
'installed:')[-1].split()[0]

to

return subprocess.check_output(cmd).decode('utf-8').partition(
'installed:')[-1].split()[0]

and above change can also run well in python3.6, so I think we should make this change to be compatible with python3.5 and python3.6 at the same time.

[1] https://docs.python.org/3.5/library/subprocess.html