[3.13] gh-127906: Fix Py_BUILD_ASSERT_EXPR() on Windows

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Change Py_BUILD_ASSERT_EXPR implementation on Windows to avoid a compiler warning about an unnamed structure.

• Issue: [tests] Backport test_cext and test_cppext (C API tests) from main to the 3.12 branch #127906

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Fix the following test_cext warning treated as an error:

 extension.c(56): error C2220: the following warning is treated as an error
extension.c(56): warning C4116: unnamed type definition in parentheses

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I tested locally that this change fix python -m test test_cext -m test_build -v -u all -m test_build_c11.

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Python 3.12 is not impacted. The main branch already contains the fix.