• unpacked Winpython64-3.9.2.0dot.exe
• start WinPython Command Prompt.exe
  pip install pyqt5-tools
  ...
  Successfully installed PyQt5-sip-12.8.1 click-7.1.2 pyqt5-5.15.2 pyqt5-plugins-5.15.2.2.0.1 pyqt5-tools-5.15.2.3.0.2 python-dotenv-0.15.0 qt5-applications-5.15.2.2.1 qt5-tools-5.15.2.1.0.1
• using Qt Assistant.exe, Qt Designer.exe or Qt Linguist.exe doesn't start anything cause the binaries are located in %WINPYDIR%\Lib\site-packages\qt5_applications\Qt\bin which is not checked in the bat files

thanks a lot @mcmetrox ! Sorry I didn't notice the pull request sooner.