Unpickling modifies original class in unexpected ways

Consider the following test. I don't use the unpickled result, so I would consider it to be side-effect free, but it's not. I don't think pickling/unpickling an object should have such obvious effects on the class.

```
def test_cloudpickle(self):
log = []

class Foo(object):
def __init__(self):
log.append("hello from Foo init")

f = Foo()

if True: # set to False to make test pass
pickled = cloudpickle.dumps(f)
unpickeled = pickle.loads(pickled)

f2 = Foo()

self.assertEqual(len(log), 2)
```