"Error while finding spec for PyQt5.uic.pyuicr" while converting -ui file to .py

[zandar]

Hi,
I'm a very beginner and I'm trying to make my first Qt application but I'm stuck with the very first step of converting the .ui file created with QT Designer into a .py file.
I'm on windows 8 pro, python 3.4.3-x32, Qt 5.4.1 x32 and PyQT 5.4.1-Py3.4-Qt5.4.1-x32, with their dirs in the system path. PYTHONPATH is c:\python34.
In a terminal window, from the directory containing the .ui file I run this command:

pyuic5 -x mainform.ui -o mainform.py

This raise the error::
C:\python34\python.exe: Error while finding spec for 'PyQt5.uic.pyuic' (<class 'ImportError'>: No module named 'PyQt.uic.port_v3')

Searching on Google for this kind of error gives only one result, so it seem quite an unusual situation.
I tried to uninstall and reinstall all (python, Qt and PyQt, in this order) but the error does persist.
Could somebody help me to solve this issue?

Thanks in advance