Ubuntu 18.04: Couldn't load SIP module qgis

I found a solution for my problem so i answered to my question to someone stuck in my situation:

According this page on Stack Overflow:

PyQt in PyPI is not compatible with the installed Qt

So I removed pyqt5 using pip3:

pip3 uninstall PyQt5

and installed PyQt5 from apt:

sudo apt-get install python3-pyqt5

Now Qgis seems to work correctly.