py/objset: Check that RHS of a binary op is a set/frozenset.

CPython docs explicitly state that the RHS of a set/frozenset binary op
must be a set to prevent user errors.  It also preserves commutativity of
the ops, eg: "abc" & set() is a TypeError, and so should be set() & "abc".

This change actually decreases unix (x64) code by 160 bytes; it increases
stm32 by 4 bytes and esp8266 by 28 bytes (but previous patch already
introduced a much large saving).

py/objset.c

@@ -463,6 +463,10 @@ STATIC mp_obj_t set_binary_op(mp_binary_op_t op, mp_obj_t lhs, mp_obj_t rhs) {
     #else
     bool update = true;
     #endif
+    if (op != MP_BINARY_OP_IN && !is_set_or_frozenset(rhs)) {
+        // For all ops except containment the RHS must be a set/frozenset
+        return MP_OBJ_NULL;
+    }
     switch (op) {
         case MP_BINARY_OP_OR:
             return set_union(lhs, rhs);