Add invoke as task runner, ruff and fix mypy errors

files/keep-files.py

@@ -4,7 +4,7 @@ import os
 import argparse
 
 
-def main():
+def main() -> None:
     parser = argparse.ArgumentParser(
         description="Retain specified number of files in a directory sorted by name, delete others."
     )
@@ -32,7 +32,7 @@ def main():
     sorted_files = sorted(files)
 
     # Identify files to delete
-    to_delete = sorted_files[:-args.keep] if args.keep > 0 else sorted_files.copy()
+    to_delete = sorted_files[: -args.keep] if args.keep > 0 else sorted_files.copy()
 
     # Delete files and print results
     for filename in to_delete: