gh-138143: Allow anonymous unions in public headers, using _Py_ANONYMOUS

[encukou]

We already use an anonymous union for PyObject. This makes the workarounds available in all public headers:

• MSVC: __pragma(warning(disable: 4201)) (with push/pop). Warning 4201 is specifically for anonymous unions, so let's disable for all of <Python.h>
• GCC/clang, pedantic old C standards: define _Py_ANONYMOUS as __extension__
• otherwise, define _Py_ANONYMOUS as nothing

(Note that this is only for public headers -- CPython internals use C11, which has anonymous structs/unions.)

C API WG vote: capi-workgroup/decisions#74

• Issue: Allow anonymous unions in public headers, with _Py_ANONYMOUS #138143

[serhiy-storchaka]

Can the warning only be silenced for anonymous unions, not for anonymous structs?

Maybe add _Py_ANONYMOUS_UNION, or even _Py_BEGIN_ANONYMOUS_UNION and _Py_END_ANONYMOUS_UNION?

[encukou]

I think that would mean more typing for little benefit.
If you put _Py_ANONYMOUS struct in the headers, tests will fail in strict C++ mode.