do not call property if referenced by class, not object

[dhalbert]

Fixes #1027.

>>> import analogio
>>> analogio.AnalogIn.value   # this used to hang or (in 2.x) return an integer
<property>
>>> import board
>>> a0 = analogio.AnalogIn(board.A0)
>>> a0.value
27200

I should probably submit this upstream as well. (I thought this code was in upstream but it is not.)

[tannewt]

Thanks!