Allow exception pickling.

[bolaum]

This fixes exception pickling. The error can be reproduced with the following code:

>>> import pickle
>>> import psutil
>>>
>>> pkexc = pickle.dumps(psutil.NoSuchProcess(123))
>>> e = pickle.loads(pkexc)
Traceback (most recent call last):
...
TypeError: __init__() takes at least 2 arguments (1 given)

Calling the base class constructor fixed it.

[giampaolo]

Nice one, thanks

[bolaum]

Wow! This was impressively fast 👍