puppetboard fails to start with scheduler enabled

[buzzdeee]

Describe the bug

Try to update the Puppetboard package for OpenBSD from 4.2.6 to 4.3.0.

With the scheduler enabled, see below related configuration options:

ENABLE_CLASS = True
# mapping between the status of the events (from PuppetDB) and the columns of the table to display
CLASS_EVENTS_STATUS_COLUMNS = [
    # ('skipped', 'Skipped'),
    ('failure', 'Failure'),
    ('success', 'Success'),
    ('noop', 'Noop'),
]

#CACHE_TYPE = 'MemcachedCache'
CACHE_TYPE = 'SimpleCache'
# Cache litefime in second
CACHE_DEFAULT_TIMEOUT = 3600

# List of scheduled jobs to trigger
#   * `id`: job's ID
#   * `func`: full path of the function to execute
#   * `trigger`: should be 'interval' if you want to run the job at regular intervals
#   * `seconds`: number of seconds between 2 triggered jobs
SCHEDULER_JOBS = [{
    'id': 'do_build_async_cache_1',
    'func': 'puppetboard.schedulers.classes:build_async_cache',
    'trigger': 'interval',
    'seconds': 300,
}]
SCHEDULER_ENABLED = True
SCHEDULER_LOCK_BIND_PORT = 49100

Puppetboard refuses to start:

/usr/local/bin/gunicorn --workers 1 puppetboard_wsgi:application 
[2023-06-13 14:07:48 +0200] [73476] [INFO] Starting gunicorn 20.0.4
[2023-06-13 14:07:48 +0200] [73476] [INFO] Listening at: http://127.0.0.1:8000 (73476)
[2023-06-13 14:07:48 +0200] [73476] [INFO] Using worker: sync
[2023-06-13 14:07:48 +0200] [86370] [INFO] Booting worker with pid: 86370
scheduler enabled
[2023-06-13 14:07:50 +0200] [86370] [ERROR] Exception in worker process
Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/apscheduler/util.py", line 290, in ref_to_obj
    obj = __import__(modulename, fromlist=[rest])
ModuleNotFoundError: No module named 'puppetboard.schedulers'

During handling of the above exception, another exception occurred:

Traceback (most recent call last):
  File "/usr/local/lib/python3.10/site-packages/gunicorn/arbiter.py", line 583, in spawn_worker
    worker.init_process()
  File "/usr/local/lib/python3.10/site-packages/gunicorn/workers/base.py", line 119, in init_process
    self.load_wsgi()
  File "/usr/local/lib/python3.10/site-packages/gunicorn/workers/base.py", line 144, in load_wsgi
    self.wsgi = self.app.wsgi()
  File "/usr/local/lib/python3.10/site-packages/gunicorn/app/base.py", line 67, in wsgi
    self.callable = self.load()
  File "/usr/local/lib/python3.10/site-packages/gunicorn/app/wsgiapp.py", line 49, in load
    return self.load_wsgiapp()
  File "/usr/local/lib/python3.10/site-packages/gunicorn/app/wsgiapp.py", line 39, in load_wsgiapp
    return util.import_app(self.app_uri)
  File "/usr/local/lib/python3.10/site-packages/gunicorn/util.py", line 358, in import_app
    mod = importlib.import_module(module)
  File "/usr/local/lib/python3.10/importlib/__init__.py", line 126, in import_module
    return _bootstrap._gcd_import(name[level:], package, level)
  File "<frozen importlib._bootstrap>", line 1050, in _gcd_import
  File "<frozen importlib._bootstrap>", line 1027, in _find_and_load
  File "<frozen importlib._bootstrap>", line 1006, in _find_and_load_unlocked
  File "<frozen importlib._bootstrap>", line 688, in _load_unlocked
  File "<frozen importlib._bootstrap_external>", line 883, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/var/www/puppetboard/puppetboard_wsgi.py", line 13, in <module>
    from puppetboard.app import app as application
  File "/var/www/puppetboard/puppetboard/app.py", line 44, in <module>
    get_scheduler()
  File "/var/www/puppetboard/puppetboard/core.py", line 114, in get_scheduler
    scheduler.init_app(app)
  File "/usr/local/lib/python3.10/site-packages/flask_apscheduler/scheduler.py", line 84, in init_app
    self._load_jobs()
  File "/usr/local/lib/python3.10/site-packages/flask_apscheduler/scheduler.py", line 336, in _load_jobs
    self.add_job(**job)
  File "/usr/local/lib/python3.10/site-packages/flask_apscheduler/scheduler.py", line 168, in add_job
    return self._scheduler.add_job(**job_def)
  File "/usr/local/lib/python3.10/site-packages/apscheduler/schedulers/base.py", line 438, in add_job
    job = Job(self, **job_kwargs)
  File "/usr/local/lib/python3.10/site-packages/apscheduler/job.py", line 49, in __init__
    self._modify(id=id or uuid4().hex, **kwargs)
  File "/usr/local/lib/python3.10/site-packages/apscheduler/job.py", line 162, in _modify
    func = ref_to_obj(func)
  File "/usr/local/lib/python3.10/site-packages/apscheduler/util.py", line 292, in ref_to_obj
    raise LookupError('Error resolving reference %s: could not import module' % ref)
LookupError: Error resolving reference puppetboard.schedulers.classes:build_async_cache: could not import module
[2023-06-13 14:07:50 +0200] [86370] [INFO] Worker exiting (pid: 86370)
[2023-06-13 14:07:50 +0200] [73476] [INFO] Shutting down: Master
[2023-06-13 14:07:50 +0200] [73476] [INFO] Reason: Worker failed to boot.

If I disable the scheduler, it starts happily.

Puppetboard version

Puppetboard 4.3.0,

Environment and installation method
python 3.10.11 on OpenBSD 7.3, amd64.