Explicitly import importlib.util to resolve mypy error (#1086)

Author: Dan-Flores

src/torchcodec/_internally_replaced_utils.py

@@ -5,6 +5,7 @@
 # LICENSE file in the root directory of this source tree.
 
 import importlib
+import importlib.util
 import sys
 from pathlib import Path
 from types import ModuleType