gh-161926: Honor overridden __float__ in complex() - #161949

Summary

The single-positional-argument form of complex() has bypassed an overridden __float__ on float subclasses since 3.14, returning the stored float value and suppressing exceptions raised by the override. Convert through PyNumber_Float(), as the keyword and two-argument forms already do.

Add regression coverage for the converted value and exception propagation, including construction of a complex subclass, and a NEWS entry.

Fixes #161926.

Testing

Built and tested on Windows x64 in Debug mode:

PCbuild\build.bat -c Debug /m:8
PCbuild\amd64\python_d.exe -m test test_complex -m '*test_constructor_float_subclass*' -v
PCbuild\amd64\python_d.exe -m test test_complex test_capi.test_complex test_cmath test_float test_builtin -j 4
PCbuild\amd64\python_d.exe -m test test_complex test_capi.test_complex -R 3:3
PCbuild\amd64\python_d.exe Tools\patchcheck\patchcheck.py

• Before: On main at 2c2d8018a20da33c749f42cc6a4ae55664a88321, the two new regression tests produce four failures: the overridden value is ignored and the expected exception is absent for both constructor classes.
• After: A Windows x64 Debug build succeeds with no warnings or errors. test_complex, test_capi.test_complex, test_cmath, test_float, and test_builtin pass, with 306 tests run and 9 skipped. test_complex and test_capi.test_complex also pass -R 3:3 reference-leak checks. Patchcheck, applicable pre-commit hooks, and git diff --check pass.

The default full suite (PCbuild\amd64\python_d.exe -m test -j 4 --timeout 180 --slowest) ran 52,079 tests with two failures, both also reproduced on an unmodified build of the same base commit. test_device_encoding depends on the console handle and passes when the whole test_os.test_os file is rerun in a terminal. test_wrong_cert_tls13 can receive a localized Windows connection-reset message that does not match its English-only assertion; the complete test_ssl file has the same failure on the base build.

• Issue: complex(x) ignores an overridden __float__ of a float subclass since 3.14 #161926