Fix tests on PyPy3.11+

[AlexWaygood]

PyPy made a recent change that exposes a __text_signature__ attribute on all functions: pypy/pypy@d3fba0e. That broke this test, which assumed that typing_extensions.NamedTuple would define __text_signature__ but typing.NamedTuple would not. But typing_extensions.NamedTuple itself hasn't defined __text_signature__ for a while, so the special-casing of __text_signature__ here is outdated. We can fix the PyPy failure by just simplifying the test and getting rid of the special casing.

Fixes #751

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.38%. Comparing base (7b79d4a) to head (4e20797).
⚠️ Report is 1 commits behind head on main.

@@           Coverage Diff           @@
##             main     #756   +/-   ##
=======================================
  Coverage   97.38%   97.38%           
=======================================
  Files           3        3           
  Lines        7773     7773           
=======================================
  Hits         7570     7570           
  Misses        203      203           

Flag	Coverage Δ
3.10	89.09% <ø> (ø)
3.10.4	89.09% <ø> (ø)
3.11	88.33% <ø> (ø)
3.11.0	87.57% <ø> (ø)
3.12	88.27% <ø> (ø)
3.12.0	88.26% <ø> (ø)
3.13	83.05% <ø> (ø)
3.13.0	83.77% <ø> (ø)
3.14	79.14% <ø> (ø)
3.9	89.79% <ø> (ø)
3.9.12	89.79% <ø> (ø)
pypy3.10	88.92% <ø> (ø)
pypy3.11	88.18% <ø> (?)
pypy3.9	89.62% <ø> (ø)

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Files with missing lines	Coverage Δ
src/test_typing_extensions.py	98.41% <ø> (ø)

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[AlexWaygood]

(We stopped setting __text_signature__ manually on NamedTuple in #267)