py/objarray: Disallow slice-assignment to read-only memoryview.

Also comes with a test for this.  Fixes issue #2904.

py/objarray.c

@@ -418,6 +418,10 @@ STATIC mp_obj_t array_subscr(mp_obj_t self_in, mp_obj_t index_in, mp_obj_t value
                 uint8_t* dest_items = o->items;
                 #if MICROPY_PY_BUILTINS_MEMORYVIEW
                 if (o->base.type == &mp_type_memoryview) {
+                    if ((o->typecode & 0x80) == 0) {
+                        // store to read-only memoryview not allowed
+                        return MP_OBJ_NULL;
+                    }
                     if (len_adj != 0) {
                         goto compat_error;
                     }

tests/basics/memoryview1.py

@@ -18,6 +18,10 @@ try:
     m[0] = 1
 except TypeError:
     print("TypeError")
+try:
+    m[0:2] = b'00'
+except TypeError:
+    print("TypeError")
 
 # test writing to bytearray
 b = bytearray(b)