Fix TypeError when handle is None on Python 3

[mnaberez]

Fixes #103. Adds an explicit check for None on the handle returned by openDev().

Python 3 changed the behavior of comparing None to int as shown below.

$ python2
>>> None <= 0
True

$ python3
>>> None <= 0
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
TypeError: '<=' not supported between instances of 'NoneType' and 'int'

[davelopez01]

Thanks for fixing this.