Add support to submodules

[yqkcn]

[yqkcn]

what does this sub module stuff will help to achieve? I can write code like this:

import kombu

kombu.serialization.pickle_protocol = 4

without this, it raise an Exception

Traceback (most recent call last):
  File "/home/yqk/Desktop/projects/yqk_test/yqk.py", line 17, in <module>
    kombu.serialization.pickle_protocol = 4
  File "/home/yqk/.local/share/virtualenvs/yqk_test-I4BoekgW/lib/python3.9/site-packages/kombu/__init__.py", line 78, in __getattr__
    return ModuleType.__getattribute__(self, name)
AttributeError: module 'kombu' has no attribute 'serialization'