Why does 'import PyQt5.QtCore` require py-sip in runtime?

[Yuri]

This import fails in python36 w/out py36-sip installed:

Traceback (most recent call last):
   File "x.py", line 5, in <module>
     import PyQt5.QtCore
ModuleNotFoundError: No module named 'sip'


py-sip description red like it should be required in the runtime.

But if it is always required for PyQt5.QtCore, why isn't it 
automatically included then?


Yuri