Replication of original PR #594: Add support for sentinels (PEP 661)

[yashwanthatla]

PR Summary

Add Sentinel class for unique typing sentinel objects

Overview

This PR adds a new Sentinel class to create unique sentinel objects for typing purposes, with special handling for Python versions before 3.11. The class is exposed as a public API and includes comprehensive test coverage.

Change Types

Type	Description
Feature	New Sentinel class for typing purposes
Test	Test cases for the new Sentinel class

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Affected Modules

Module / File	Change Description
typing_extensions.py	Added new Sentinel class and exposed it in __all__
test_typing_extensions.py	Added import and test cases for Sentinel class

Notes for Reviewers

• Special handling is implemented for Python versions before 3.11
• Tests cover representation, type expression unions, non-callable behavior, and non-picklable behavior