Restrict pickle class loading for .localstate deserialization

[praegt]

Summary

Replace the unvalidated cPickle.loads() call in LocalState.loads() with a _SafeUnpickler that uses a strict class allowlist. Only LocalState, EnvvarCollector, and set are permitted during deserialization. Any other class raises UnpicklingError, which is caught and returns an empty LocalState.

The serialization format remains pickle, so there is no behavioral change for existing .localstate files.

Testing

• Unit tests: 8/8 pass (including new test_loads_rejects_unlisted_class and test_dumps_loads_roundtrip)
• Broader suite: 823 passed, 1 skipped across tests/unit/containers/ and tests/unit/operations/
• E2E: Verified with actual eb local printenv and eb local setenv commands

Release

Includes version bump to 3.27.2.