plugins: add annotations to _list_plugin_filenames

sopel/plugins/__init__.py

@@ -33,6 +33,7 @@
 import itertools
 import logging
 import os
+from typing import TYPE_CHECKING
 
 # TODO: use stdlib importlib.metadata when possible, after dropping py3.9.
 # Stdlib does not support `entry_points(group='filter')` until py3.10, but
@@ -42,10 +43,13 @@
 
 from . import exceptions, handlers, rules  # noqa
 
+if TYPE_CHECKING:
+    from collections.abc import Iterable
+
 
 LOGGER = logging.getLogger(__name__)
 
-def _list_plugin_filenames(directory):
+def _list_plugin_filenames(directory: str | os.PathLike) -> Iterable[tuple[str, str]]:
     # list plugin filenames from a directory
     # yield 2-value tuples: (name, absolute path)
     base = os.path.abspath(directory)