Fix error message when subclassing TypeVarTuple

[JanEricNitschke]

Issue discovered by running CPython 3.12 test_typing.py on typing_extensions: https://github.com/JanEricNitschke/typing_extensions/actions/runs/17692936537/job/50288896717#step:7:120

Test from here: https://github.com/python/cpython/blob/main/Lib/test/test_typing.py#L1526

Regex from here (adjusted to also handle typing_extensions): https://github.com/python/cpython/blob/main/Lib/test/test_typing.py#L61

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.39%. Comparing base (f7dbe27) to head (b088f82).

@@            Coverage Diff             @@
##             main     #679      +/-   ##
==========================================
- Coverage   97.39%   97.39%   -0.01%     
==========================================
  Files           3        3              
  Lines        7767     7793      +26     
==========================================
+ Hits         7565     7590      +25     
- Misses        202      203       +1     

Flag	Coverage Δ
3.10	89.06% <85.71%> (-0.02%)	⬇️
3.10.4	89.06% <85.71%> (-0.02%)	⬇️
3.11	88.32% <89.28%> (+<0.01%)	⬆️
3.11.0	87.56% <89.28%> (+<0.01%)	⬆️
3.12	88.28% <89.28%> (+0.01%)	⬆️
3.12.0	88.27% <89.28%> (+0.01%)	⬆️
3.13	81.82% <75.00%> (-0.01%)	⬇️
3.13.0	82.54% <75.00%> (-0.01%)	⬇️
3.14	78.28% <75.00%> (+<0.01%)	⬆️
3.9	89.77% <85.71%> (-0.02%)	⬇️
3.9.12	89.77% <85.71%> (-0.02%)	⬇️
pypy3.10	88.90% <85.71%> (-0.02%)	⬇️
pypy3.11	88.18% <89.28%> (+<0.01%)	⬆️
pypy3.9	89.60% <85.71%> (-0.02%)	⬇️

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Files with missing lines	Coverage Δ
src/test_typing_extensions.py	98.39% <100.00%> (-0.02%)	⬇️
src/typing_extensions.py	94.00% <100.00%> (+0.01%)	⬆️

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[JelleZijlstra]

@copilot resolve the merge conflicts in this pull request

[JanEricNitschke]

Anything i can/should still do here? Conflict is only in the changelog.