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

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.39%. Comparing base (72895d0) to head (93aade6).

@@           Coverage Diff           @@
##             main     #757   +/-   ##
=======================================
  Coverage   97.38%   97.39%           
=======================================
  Files           3        3           
  Lines        7773     7785   +12     
=======================================
+ Hits         7570     7582   +12     
  Misses        203      203           

Flag	Coverage Δ
3.10	89.10% <100.00%> (+0.01%)	⬆️
3.10.4	89.10% <100.00%> (+0.01%)	⬆️
3.11	88.34% <100.00%> (+0.01%)	⬆️
3.11.0	87.59% <100.00%> (+0.01%)	⬆️
3.12	88.29% <100.00%> (+0.01%)	⬆️
3.12.0	88.28% <100.00%> (+0.01%)	⬆️
3.13	83.08% <100.00%> (+0.02%)	⬆️
3.13.0	83.80% <100.00%> (+0.02%)	⬆️
3.14	79.17% <100.00%> (+0.03%)	⬆️
3.9	89.81% <100.00%> (+0.01%)	⬆️
3.9.12	89.81% <100.00%> (+0.01%)	⬆️
pypy3.10	88.94% <100.00%> (+0.01%)	⬆️
pypy3.11	88.20% <100.00%> (+0.01%)	⬆️
pypy3.9	89.64% <100.00%> (+0.01%)	⬆️

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Files with missing lines	Coverage Δ
src/test_typing_extensions.py	98.41% <100.00%> (+<0.01%)	⬆️
src/typing_extensions.py	93.95% <100.00%> (+0.02%)	⬆️

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