[3.13] pythongh-127906: Fix Py_BUILD_ASSERT_EXPR() on Windows (python#127994)

pythongh-127906: Fix Py_BUILD_ASSERT_EXPR() on Windows

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

Author: vstinner

Include/pymacro.h

@@ -47,7 +47,7 @@
 #define Py_CHARMASK(c) ((unsigned char)((c) & 0xff))
 
 #if (defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L \
-     && !defined(__cplusplus))
+     && !defined(__cplusplus) && !defined(_MSC_VER))
 #  define Py_BUILD_ASSERT_EXPR(cond) \
     ((void)sizeof(struct { int dummy; _Static_assert(cond, #cond); }), \
      0)