Update safe_extractor.py fix lint errors

additionally converted `_is_within_directory` to static method

luigi/safe_extractor.py

@@ -16,11 +16,11 @@
 #
 
 """
-This module provides a class `SafeExtractor` that offers a secure way to extract tar files while 
-mitigating path traversal vulnerabilities, which can occur when files inside the archive are 
+This module provides a class `SafeExtractor` that offers a secure way to extract tar files while
+mitigating path traversal vulnerabilities, which can occur when files inside the archive are
 crafted to escape the intended extraction directory.
 
-The `SafeExtractor` ensures that the extracted file paths are validated before extraction to 
+The `SafeExtractor` ensures that the extracted file paths are validated before extraction to
 prevent malicious archives from extracting files outside the intended directory.
 
 Classes:
@@ -60,7 +60,8 @@ def __init__(self, path="."):
         """
         self.path = path
 
-    def _is_within_directory(self, directory, target):
+    @staticmethod
+    def _is_within_directory(directory, target):
         """
         Checks if a target path is within a given directory.