Installing PySide/ShiBoken on Ubuntu 20.04

Question

I've been trying to get PySide (more specifically, shiboken, PySide's dependancy) installed in a venv on Ubuntu 20.04 (Focal). However, CMake seems to be having trouble finding Qt.

...
-- Found unsuitable Qt version "5.12.8" from /usr/bin/qmake
    -- Found PythonLibs: /usr/lib/x86_64-linux-gnu/libpython2.7.so (found suitable version "2.7.18", minimum required is "2.6")
    -- Found LibXml2: /usr/lib/x86_64-linux-gnu/libxml2.so (found suitable version "2.9.10", minimum required is "2.6.32")
    -- Found LibXslt: /usr/lib/x86_64-linux-gnu/libxslt.so (found suitable version "1.1.34", minimum required is "1.1.19")
    Qt QTGUI library not found.
    Qt QTXML library not found.
    Qt QTCORE library not found.
    CMake Error at ApiExtractor/CMakeLists.txt:82 (qt4_add_resources):
      Unknown CMake command "qt4_add_resources".
    
    
    -- Configuring incomplete, errors occurred!
    See also "/tmp/pip-install-stnnA9/shiboken/shiboken_build/py2.7-qt5.12.8-64bit-release/shiboken/CMakeFiles/CMakeOutput.log".
    error: Error configuring shiboken

I have Qt5 installed, and it seems that shiboken wants Qt4? Is there a way I can make it play nice with Qt5? If that's not possible, how do I downgrade to Qt4?