Modernise cron configuration import mechanics

As far as I can tell this is functionally equivalent to the previous
approach, however uses a mixture of supported Python and Django
import utils rather than relying on `__import__` (which is discouraged)
and the deprecated `imp` module.

This resolves deprecation warnings as well as some resource warnings
that originated from the previous `imp.find_module` call.

Author: PeterJCLaw

django_lightweight_queue/cron_scheduler.py

@@ -1,14 +1,15 @@
 import re
-import imp
 import time
 import datetime
+import importlib
 import threading
 from typing import Any, cast, Dict, List, Tuple, Callable, Optional, Sequence
 
 from typing_extensions import TypedDict
 
 from django.apps import apps
 from django.core.management import call_command
+from django.utils.module_loading import module_has_submodule
 
 from .task import task
 from .types import QueueName
@@ -132,22 +133,16 @@ def get_matcher(minval, maxval, t):
         return lambda x: x in t_parts
 
     for app_config in apps.get_app_configs():
-        app = app_config.name
+        # Adapted from django.utils.module_loading.autodiscover_modules
         try:
-            # __import__ will break with anything other than a str object(!),
-            # including e.g. unicode. So force to a str.
-            part = str(app.split('.')[-1])
-
-            app_path = __import__(app, {}, {}, [part]).__path__
-        except AttributeError:
-            continue
-
-        try:
-            imp.find_module('cron', app_path)
-        except ImportError:
-            continue
-
-        mod = __import__('{}.cron'.format(app), fromlist=(app,))
+            mod = importlib.import_module(f'{app_config.name}.cron')
+        except Exception:
+            if module_has_submodule(app_config.module, 'cron'):
+                # The module exists, so the error was something it did -- bubble the error.
+                raise
+            else:
+                # No module, move on.
+                continue
 
         for row in cast(List[CronConfig], mod.CONFIG):
             row['min_matcher'] = get_matcher(0, 59, row.get('minutes'))