gh-138143: Allow anonymous unions in public headers, using _Py_ANONYMOUS (GH-137283)

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: https://github.com/capi-workgroup/decisions/issues/74

Include/Python.h

@@ -60,6 +60,15 @@
 #  endif
 #endif // Py_GIL_DISABLED
 
+#ifdef _MSC_VER
+// Ignore MSC warning C4201: "nonstandard extension used: nameless
+// struct/union".  (Only generated for C standard versions less than C11, which
+// we don't *officially* support.)
+__pragma(warning(push))
+__pragma(warning(disable: 4201))
+#endif
+
+
 // Include Python header files
 #include "pyport.h"
 #include "pymacro.h"
@@ -139,4 +148,8 @@
 #include "cpython/pyfpe.h"
 #include "cpython/tracemalloc.h"
 
+#ifdef _MSC_VER
+__pragma(warning(pop))  // warning(disable: 4201)
+#endif
+
 #endif /* !Py_PYTHON_H */