title: QButtonGroup not making checkboxes exclusive
body: <p>I am attempting to make a set of exclusive checkboxes, using QGroupBox (which, as I understand it, is exclusive by default), but when I run my program, the checkboxes are not exclusive and behave as they normally would.</p>
  
  <pre><code>skillP = QCheckBox("Passive")
  skillCb = QCheckBox("Combat")
  skillCm = QCheckBox("Command")
  skillP.setChecked(True)
  addskillG = QButtonGroup()
  addskillG.addButton(skillP)
  addskillG.addButton(skillCm)
  addskillG.addButton(skillCb)
  </code></pre>
  
  <p>Is there anything I'm doing wrong?</p>