I think it'd be useful to automatically unexport previously used pins.
Since a pin is in your case a normal Python object of Pin class its __del__ method
can be used to perform this task. This way pin will be freed during object's destruction.

Something along these lines;

 class Pin(object):
     # [...]
     def __del__(self):
         self.unexport()

There is obviously an issue when sb creates multiple instances of Pin objects referencing the same hardware pin.