OK, reverting PyQt5 (originally version 5.10) to an earlier version fixed the problem
pip uninstall PyQt5
pip install PyQt5==5.9.2

Interesting: do you mean PyQt5-5.10 needs Windows 10 ?

move back to PyQt5-5.9.2 as Spyder and Bluetooth don't like PyQt-5.10.0

Mouse cursor issue with PyQt 5.10 spyder-ide/spyder#6393
Planned to be fixed in Spyder v3.2.7. However, see the developer's comment: "There has been several reports that things are failing strangely with 5.10."

Spyder v3.2.7 now requires PyQt older than v5.10.

import PyQt5.Qt imports everything including bluetooth. While that import looks nice for the sake of only having one it does cost in scenarios such as this. So, just use the regular imports like import PyQt5.QtWidgets instead.

spyder-ide/qtpy#143 was released with Spyder 1.4.1.

Aaaaaaargh. So WinPython embedded version will be updated from QtPy-1.3.0 to QtPy-1.42.
Thank you @altendky

User in needs can copy ther copy of QtPy in
\Lib\site-packages\qtpy
to
\Lib\site-packages\winpython_vendor\qtpy

Win8.1 cannot start cellpose because of the PyQt5 compatibility problem,

The solution is :

change
from PyQt5.Qt import * to

import PyQt5
from PyQt5 import *

in any place of cellpose code that import PyQt5.

Another solution is renamed file C:\Python\Python38-32\Lib\site-packages\PyQt5\Qt\bin\Qt5Bluetooth.dll to _Qt5Bluetooth.dll.

More details see:

https://stackoverflow.com/questions/55205708/why-pyqt5-import-show-a-error-about-windows-and-bluetooth

and what are the said places ?
Microsoft support for Windows 8.1 will end on January 10, 2023. so:

• you can proposing a patch in all bad packages with the problem, that impacts you,
• or it's a bit of helping yourself with a nice 'replace' in all problematic source files 'live'
• or just displace the dll for your users.