Support the new name of sip module that is used since PyQt v5.11.2

For details, see:

• https://www.riverbankcomputing.com/static/Docs/PyQt5/incompatibilities.html#importing-the-sip-module
• https://bugs.debian.org/cgi-bin/bugreport.cgi?bug=966036

I have verified that on my system the PyQt5.sip module (from PyQt 5.15.2 Debian package) contains all names that are used in Kajongg:

>>> from PyQt5 import sip
>>> sip.unwrapinstance 
<built-in function unwrapinstance>
>>> sip.SIP_VERSION_STR
'5.4.0'
>>> sip.cast
<built-in function cast>

Note for future: in PyQt6 the sip module is named PyQt6.sip.