byte[] AKA IDL sequence<octet> fields required to be sequence of bytes instances instead of bytes instance

Bug report

Required Info:

• Operating System:
  • focal
• Installation type:
  • source
• Version or commit hash:
  • master
• DDS implementation:
  • n/a
• Client library (if applicable):
  • rclpy

Steps to reproduce issue

from test_msgs.msg import UnboundedSequences
msg = UnboundedSequences()
msg.byte_values = b'ffff'

Expected behavior

A bytes instance should be accepted, as it is the type documented in the design doc.

http://design.ros2.org/articles/idl_interface_definition.html

Actual behavior

>>> from test_msgs.msg import UnboundedSequences
>>> msg = UnboundedSequences()
>>> msg.byte_values = msg.byte_values = b'ffff'
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/home/sloretz/ws/ros2/install/test_msgs/lib/python3.8/site-packages/test_msgs/msg/_unbounded_sequences.py", line 466, in byte_values
    assert \
AssertionError: The 'byte_values' field must be a set or sequence and each value of type 'bytes'

It only accepts a sequence of bytes instances

msg.byte_values = (b'ff', b'ff')

Additional information

I noticed while reviewing #129

Start with the generated setter in /home/sloretz/ws/ros2/install/test_msgs/lib/python3.8/site-packages/test_msgs/msg/_unbounded_sequences.py and compare its byte_values validation with the IDL design document. Reproduce the bytes assignment shown in the issue and verify that a bytes instance is accepted while the existing sequence form remains valid. Pull request #142 is already open, so check its state before starting work.