fixes#7198
We recently bumped the bundled version of PyQt (5.14.2->5.15.4).
It seems the new version has a lot more dynamic dependencies.
From https://doc.qt.io/qt-5/linux-requirements.html :
> From Qt 5.15 onwards, Qt does require libxcb 1.11. Also, the -qt-xcb
> configure option got removed that was bundling some of the libs below.
We are using the prebuilt wheels for PyQt5 from PyPI. Presumably those
had previously been built using the -qt-xcb option.