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[bot]]

Codecov Report

:white_check_mark: All modified and coverable lines are covered by tests. :white_check_mark: Project coverage is 97.36%. Comparing base (9215c95) to head (a49d36d).

@@            Coverage Diff             @@
##             main     #679      +/-   ##
==========================================
- Coverage   97.36%   97.36%   -0.01%     
==========================================
  Files           3        3              
  Lines        7680     7706      +26     
==========================================
+ Hits         7478     7503      +25     
- Misses        202      203       +1     

Flag	Coverage Δ
3.10	88.98% <85.71%> (-0.02%)	:arrow_down:
3.10.4	88.98% <85.71%> (-0.02%)	:arrow_down:
3.11	88.22% <89.28%> (?)
3.11.0	87.46% <89.28%> (+<0.01%)	:arrow_up:
3.12	88.18% <89.28%> (+0.01%)	:arrow_up:
3.12.0	88.17% <89.28%> (+0.01%)	:arrow_up:
3.13	82.38% <75.00%> (?)
3.13.0	82.38% <75.00%> (-0.01%)	:arrow_down:
3.14	78.84% <75.00%> (+<0.01%)	:arrow_up:
3.9	89.69% <85.71%> (-0.02%)	:arrow_down:
3.9.12	89.69% <85.71%> (-0.02%)	:arrow_down:
pypy3.10	88.81% <85.71%> (-0.02%)	:arrow_down:
pypy3.11	88.08% <89.28%> (+<0.01%)	:arrow_up:
pypy3.9	89.52% <85.71%> (-0.02%)	:arrow_down:

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Files with missing lines	Coverage Δ
src/test_typing_extensions.py	98.38% <100.00%> (-0.02%)	:arrow_down:
src/typing_extensions.py	93.90% <100.00%> (+0.01%)	:arrow_up:

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