Separate the FileWatcher filtering function

Separate the function to filter events so it is easier to test
separately.

Also remove an unnecessary `type: ignore` whole at it.

Signed-off-by: Leandro Lucarella <[email protected]>

Author: llucax

src/frequenz/channels/util/_file_watcher.py

@@ -46,16 +46,26 @@ def __init__(
             for path in paths
         ]
         self._awatch = awatch(
-            *self._paths,
-            stop_event=self._stop_event,
-            watch_filter=lambda change, path_str: (
-                change in [event_type.value for event_type in event_types]  # type: ignore
-                and pathlib.Path(path_str).is_file()
-            ),
+            *self._paths, stop_event=self._stop_event, watch_filter=self._filter_events
         )
         self._awatch_stopped_exc: Optional[Exception] = None
         self._changes: Set[FileChange] = set()
 
+    def _filter_events(self, change: Change, path: str) -> bool:
+        """Filter events based on the event type and path.
+
+        Args:
+            change: The type of change to be notified.
+            path: The path of the file that changed.
+
+        Returns:
+            Whether the event should be notified.
+        """
+        return (
+            change in [event_type.value for event_type in self.event_types]
+            and pathlib.Path(path).is_file()
+        )
+
     def __del__(self) -> None:
         """Cleanup registered watches.