Add _frozen_importlib, _frozen_importlib_external, _imp to UNSAFE_IMPORTS

They allowed pickles to obtain live module objects to patch them, load
scripts or native extensions without triggering UnsafeImports.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: thomas-chauchefoin-tob

fickling/fickle.py

@@ -64,6 +64,9 @@
         "eval",
         # Import manipulation
         "importlib",
+        "_frozen_importlib",
+        "_frozen_importlib_external",
+        "_imp",
         "pkgutil",
         "zipimport",
         # Operator module bypasses