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/object.h

@@ -123,18 +123,7 @@ whose size is determined when the object is allocated.
   /* PyObject is opaque */
 #elif !defined(Py_GIL_DISABLED)
 struct _object {
-#if (defined(__GNUC__) || defined(__clang__)) \
-        && !(defined __STDC_VERSION__ && __STDC_VERSION__ >= 201112L)
-    // On C99 and older, anonymous union is a GCC and clang extension
-    __extension__
-#endif
-#ifdef _MSC_VER
-    // Ignore MSC warning C4201: "nonstandard extension used:
-    // nameless struct/union"
-    __pragma(warning(push))
-    __pragma(warning(disable: 4201))
-#endif
-    union {
+    _Py_ANONYMOUS union {
 #if SIZEOF_VOID_P > 4
         PY_INT64_T ob_refcnt_full; /* This field is needed for efficient initialization with Clang on ARM */
         struct {
@@ -153,9 +142,6 @@ struct _object {
 #endif
         _Py_ALIGNED_DEF(_PyObject_MIN_ALIGNMENT, char) _aligner;
     };
-#ifdef _MSC_VER
-    __pragma(warning(pop))
-#endif
 
     PyTypeObject *ob_type;
 };