gh-121249: fix naming of struct tagPyCArgObject members (#132863)

It seems, no code actually uses these names, only sizes of the unnamed
union members are important.  Though, I think it's good to be here
consistent wrt type codes ('g' for long double, etc).

This amends 85f89cb.

Author: skirpichev

Modules/_ctypes/ctypes.h

@@ -489,14 +489,14 @@ struct tagPyCArgObject {
         int i;
         long l;
         long long q;
-        long double D;
+        long double g;
         double d;
         float f;
         void *p;
 #if defined(Py_HAVE_C_COMPLEX) && defined(Py_FFI_SUPPORT_C_COMPLEX)
-        double complex C;
-        float complex E;
-        long double complex F;
+        double complex D;
+        float complex F;
+        long double complex G;
 #endif
     } value;
     PyObject *obj;