py/objarray: Add decode method to bytearray.

Reuse the implementation for bytes since it works the same way regardless
of the underlying type.  This method gets added for CPython compatibility
of bytearray, but to keep the code simple and small array.array now also
has a working decode method, which is non-standard but doesn't hurt.

py/objarray.c

@@ -530,6 +530,9 @@ STATIC mp_int_t array_get_buffer(mp_obj_t o_in, mp_buffer_info_t *bufinfo, mp_ui
 STATIC const mp_rom_map_elem_t array_locals_dict_table[] = {
     { MP_ROM_QSTR(MP_QSTR_append), MP_ROM_PTR(&array_append_obj) },
     { MP_ROM_QSTR(MP_QSTR_extend), MP_ROM_PTR(&array_extend_obj) },
+    #if MICROPY_CPYTHON_COMPAT
+    { MP_ROM_QSTR(MP_QSTR_decode), MP_ROM_PTR(&bytes_decode_obj) },
+    #endif
 };
 
 STATIC MP_DEFINE_CONST_DICT(array_locals_dict, array_locals_dict_table);

py/objstr.h

@@ -102,5 +102,6 @@ MP_DECLARE_CONST_FUN_OBJ_1(str_isalpha_obj);
 MP_DECLARE_CONST_FUN_OBJ_1(str_isdigit_obj);
 MP_DECLARE_CONST_FUN_OBJ_1(str_isupper_obj);
 MP_DECLARE_CONST_FUN_OBJ_1(str_islower_obj);
+MP_DECLARE_CONST_FUN_OBJ_VAR_BETWEEN(bytes_decode_obj);
 
 #endif // MICROPY_INCLUDED_PY_OBJSTR_H

tests/basics/bytearray_decode.py

+try:
+    print(bytearray(b'').decode())
+    print(bytearray(b'abc').decode())
+except AttributeError:
+    print("SKIP")
+    raise SystemExit