Summary

This PR adds pickle support for typing_extensions.Sentinel objects, fixing #720.

Sentinel objects now support pickling/unpickling using a module-level registry keyed by (module_name, name). Unpickled sentinels preserve object identity (is checks pass).

Changes

• Add _sentinel_registry dict for singleton tracking
• Convert __init__ to __new__ with registry-based singleton pattern
• Replace __getstate__ (which raised TypeError) with __reduce__
• __reduce__ returns (cls, (name, None, module_name)) for reconstruction
• Auto-detect calling module via inspect.currentframe()
• Update tests: verify pickle roundtrip, identity preservation, singleton behavior

Approach

This implements the singleton registry pattern discussed in the issue and consistent with the PEP 661 reference implementation. When a sentinel is created, it is registered in _sentinel_registry keyed by (module_name, name). On unpickling, Sentinel(name, module_name=module_name) is called, which looks up the existing singleton from the registry rather than creating a new object.

Motivation

As noted in #720, pickle support is expressed as a motivation in PEP 661. The current explicit rejection (__getstate__ raises TypeError) blocks legitimate use cases where sentinel values need to be serialized. Singletons are inherently forward-compatible — they simply reference an object by its qualified name and do not care what that object is later replaced with.

Tests

All 554 existing tests pass (13 skipped for version-specific features). New tests added:

• test_sentinel_picklable — verifies pickle roundtrip across all protocols
• test_sentinel_pickle_preserves_identity — verifies is checks pass after unpickling
• test_sentinel_singleton — verifies same-name sentinels return the same object