add a small .wait() after terminating player process

Avoids python3.8/subprocess.py:942:
ResourceWarning: subprocess 90387 is still running
  _warn("subprocess %s is still running" % self.pid,

qt/aqt/sound.py

@@ -312,6 +312,7 @@ class SimpleProcessPlayer(Player):  # pylint: disable=abstract-method
             # should we abort playing?
             if self._terminate_flag:
                 self._process.terminate()
+                self._process.wait(1)
                 self._process = None
                 return