added type hint (#4070)

python-package/lightgbm/libpath.py

@@ -2,9 +2,10 @@
 """Find the path to LightGBM dynamic library files."""
 import os
 from platform import system
+from typing import List
 
 
-def find_lib_path():
+def find_lib_path() -> List[str]:
     """Find the path to LightGBM library files.
 
     Returns