Juju-lint is incompatible with juju 3.1

[Bas de Bruijne]

During the release testing of juju 3.1 we found out that juju-lint is incompatible with juju 3.1. In the logs we see:

```
2023-01-30 14:37:51 ERROR unit.juju-lint/0.juju-log server.go:325 auto_lint.py timeout with this output:
 ERROR:asyncio:Task exception was never retrieved
future: <Task finished name='Task-13' coro=<Model._watch.<locals>._all_watcher() done, defined at /var/lib/juju/agents/unit-juju-lint-0/charm/venv/juju/model.py:1189> exception=JujuAPIError('client incompatible with server 3.1-rc1')>
Traceback (most recent call last):
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/juju/model.py", line 1192, in _all_watcher
    model_config = await self.get_config()
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/juju/model.py", line 2182, in get_config
    result = await config_facade.ModelGet()
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/juju/client/facade.py", line 481, in wrapper
    reply = await f(*args, **kwargs)
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/juju/client/_client3.py", line 2142, in ModelGet
    reply = await self.rpc(msg)
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/juju/client/facade.py", line 654, in rpc
    result = await self.connection.rpc(msg, encoder=TypeEncoder)
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/juju/client/connection.py", line 638, in rpc
    raise errors.JujuAPIError(result)
juju.errors.JujuAPIError: client incompatible with server 3.1-rc1

2023-01-30 14:37:51 ERROR unit.juju-lint/0.juju-log server.go:325 Uncaught exception while in charm code:
Traceback (most recent call last):
  File "./src/charm.py", line 125, in <module>
    main(JujuLintCharm)
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/ops/main.py", line 438, in main
    _emit_charm_event(charm, dispatcher.event_name)
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/ops/main.py", line 150, in _emit_charm_event
    event_to_emit.emit(*args, **kwargs)
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/ops/framework.py", line 355, in emit
    framework._emit(event) # noqa
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/ops/framework.py", line 856, in _emit
    self._reemit(event_path)
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/venv/ops/framework.py", line 931, in _reemit
    custom_handler(event)
  File "./src/charm.py", line 91, in on_config_changed
    self.helper.run_auto_lint()
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/lib/lib_jujulint.py", line 189, in run_auto_lint
    raise error
  File "/var/lib/juju/agents/unit-juju-lint-0/charm/lib/lib_jujulint.py", line 174, in run_auto_lint
    subprocess.check_output(
  File "/usr/lib/python3.8/subprocess.py", line 415, in check_output
    return run(*popenargs, stdout=PIPE, timeout=timeout, check=True,
  File "/usr/lib/python3.8/subprocess.py", line 495, in run
    stdout, stderr = process.communicate(input, timeout=timeout)
  File "/usr/lib/python3.8/subprocess.py", line 1028, in communicate
    stdout, stderr = self._communicate(input, endtime, timeout)
  File "/usr/lib/python3.8/subprocess.py", line 1869, in _communicate
    self._check_timeout(endtime, orig_timeout, stdout, stderr)
  File "/usr/lib/python3.8/subprocess.py", line 1072, in _check_timeout
    raise TimeoutExpired(
subprocess.TimeoutExpired: Command '['/usr/local/bin/auto_lint.py']' timed out after 60 seconds
```

According to the juju team, python-libjuju currently has no releases that are compatible with juju 3.1, but the master branch is. When juju 3.1 is released, python -libjuju will release as well. At that point the juju-lint charm can be rebuild and it should work again.

[Andrea Ieri (aieri)]

tentatively adding this to the juju-lint project as we are planning to shift all libjuju code to the snap