fix bug with checking file

The bug appear when the file exists or doesn't, but we don't have
permissions to read it. This one must be treated as missed permission

src/ahriman/core/triggers/trigger_loader.py

@@ -19,6 +19,7 @@
 #
 import importlib
 import logging
+import os
 
 from pathlib import Path
 from types import ModuleType
@@ -123,7 +124,8 @@ class TriggerLoader:
         *package_path_parts, class_name = module_path.split(".")
         package_or_path = ".".join(package_path_parts)
 
-        if Path(package_or_path).is_file():
+        # it works for both missing permission and file does not exist
+        if os.access(Path(package_or_path), os.R_OK):
             module = self._load_module_from_file(package_or_path, class_name)
         else:
             module = self._load_module_from_package(package_or_path)