Update sentinel

[zedzhen]

#742 (comment)
close #742

[python-cla-bot]

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[HexDecimal]

I assume the repr keyword should be added to the sentinel docs at doc/index.rst, and the versionchanged directive should be updated to clarify that only positional repr is deprecated.

[HexDecimal]

The typing.Optional[str] parameter not acknowledging None is invalid and also breaks backwards compatibility.

[codecov]

Codecov Report

❌ Patch coverage is 91.66667% with 2 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.19%. Comparing base (404b33b) to head (3c420b0).

Files with missing lines	Patch %	Lines
src/test_typing_extensions.py	83.33%	2 Missing ⚠️

@@            Coverage Diff             @@
##             main     #757      +/-   ##
==========================================
- Coverage   97.21%   97.19%   -0.02%     
==========================================
  Files           3        3              
  Lines        7797     7815      +18     
==========================================
+ Hits         7580     7596      +16     
- Misses        217      219       +2     

Flag	Coverage Δ
3.10	88.94% <91.66%> (-0.01%)	⬇️
3.10.4	88.94% <91.66%> (-0.01%)	⬇️
3.11	88.18% <91.66%> (+<0.01%)	⬆️
3.11.0	87.43% <91.66%> (+<0.01%)	⬆️
3.12	88.13% <91.66%> (+<0.01%)	⬆️
3.12.0	88.12% <91.66%> (+<0.01%)	⬆️
3.13	82.94% <91.66%> (+0.01%)	⬆️
3.13.0	83.65% <91.66%> (+0.01%)	⬆️
3.14	79.04% <91.66%> (+0.02%)	⬆️
3.9	89.64% <91.66%> (-0.01%)	⬇️
3.9.12	89.64% <91.66%> (-0.01%)	⬇️
pypy3.10	88.77% <91.66%> (+<0.01%)	⬆️
pypy3.11	88.04% <91.66%> (+<0.01%)	⬆️
pypy3.9	89.47% <91.66%> (-0.01%)	⬇️

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Files with missing lines	Coverage Δ
src/typing_extensions.py	93.95% <100.00%> (+0.03%)	⬆️
src/test_typing_extensions.py	98.15% <83.33%> (-0.04%)	⬇️

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[zedzhen]

@JelleZijlstra, please review this PR.