Symlink the systemwide PyQt/sip into the venv.

def link_pyqt(sys_python, venv_python):
    """Symlink the systemwide PyQt/sip into the venv."""
    real_site = site_dir(sys_python)
    venv_site = site_dir(venv_python)

    for f in ['sip.so', 'PyQt5']:
        (venv_site/f).symlink_to(real_site/f)