Extend fallthrough safety to LLVM in gmpy2_mpmath.c

The current fallthrough comments only suppress warnings for GCC. `__attribute__((fallthrough));` works for both GCC (>=7) and LLVM. C23 will introduce `[[fallthrough]]`.

src/gmpy2_mpmath.c

@@ -287,17 +287,17 @@ Pympz_mpmath_create_fast(PyObject *self, PyObject *const *args, Py_ssize_t nargs
     switch (nargs) {
         case 4:
             rnd = PyString_1Char(args[3]);
-            /* fallthrough */
+            __attribute__((fallthrough));
         case 3:
             prec = GMPy_Integer_AsLong(args[2]);
             if (prec == (mp_bitcnt_t)(-1)) {
                 VALUE_ERROR("could not convert prec to positive int");
                 return NULL;
             }
-            /* fallthrough */
+            __attribute__((fallthrough));
         case 2:
             exp = args[1];
-            /* fallthrough */
+            __attribute__((fallthrough));
         case 1:
             man = GMPy_MPZ_From_Integer(args[0], NULL);
             if (!man) {