Mistaken `TimeoutError` package while handling timeouts in `run_with_timeout`

[Ksawery Dziekoński]

Whenever a run-action timeout is hit, `run_with_timeout` expects to be thrown a `TimeoutError` from `concurrent.futures`, however the executing worker comes from `asyncio`, causing the exception to unexpectedly fall through.

Example traceback is a result of `gzip` taking too much time to compress a MySQL dump, due to running on a single thread.

Traceback (most recent call last):
  File "/var/lib/jujubackupall/auto_backup.py", line 156, in <module>
    auto_backup.run()
  File "/var/lib/jujubackupall/auto_backup.py", line 130, in run
    backup_results = self.perform_backup()
  File "/var/lib/jujubackupall/auto_backup.py", line 70, in perform_backup
    backup_results = backup_processor.process_backups()
  File "/var/lib/juju/agents/unit-juju-backup-all-0/charm/venv/jujubackupall/process.py", line 101, in process_backups
    controller_processor.backup_models()
  File "/var/lib/juju/agents/unit-juju-backup-all-0/charm/venv/jujubackupall/process.py", line 140, in backup_models
    self.backup_apps(JujuModel(name=model_name, model=model))
  File "/var/lib/juju/agents/unit-juju-backup-all-0/charm/venv/jujubackupall/process.py", line 148, in backup_apps
    self.backup_app(app=app, app_name=app_name, charm_name=charm_name, model_name=model_name)
  File "/var/lib/juju/agents/unit-juju-backup-all-0/charm/venv/jujubackupall/process.py", line 155, in backup_app
    charm_backup_instance.backup()
  File "/var/lib/juju/agents/unit-juju-backup-all-0/charm/venv/jujubackupall/backup.py", line 83, in backup
    action_output = check_output_unit_action(self.unit, self.backup_action_name)
  File "/var/lib/juju/agents/unit-juju-backup-all-0/charm/venv/jujubackupall/utils.py", line 95, in check_output_unit_action
    run_with_timeout(backup_action.wait(), action_name)
  File "/var/lib/juju/agents/unit-juju-backup-all-0/charm/venv/jujubackupall/utils.py", line 141, in run_with_timeout
    return run_async(wait_for(coroutine, timeout))
  File "/var/lib/juju/agents/unit-juju-backup-all-0/charm/venv/juju/loop.py", line 38, in run
    raise task.exception()
  File "/usr/lib/python3.8/asyncio/tasks.py", line 501, in wait_for
    raise exceptions.TimeoutError()
asyncio.exceptions.TimeoutError