Unable to reference global variables in serialized functions

When pickling a function that references a global variable, the global variable is no longer referenced.

```

import cloudpickle

global x
x = 123

def increment_x():
global x
x += 1
return x

copy_of_increment_x = cloudpickle.loads(cloudpickle.dumps(increment_x))

print('Expecting 124 - got ' + str(increment_x()))
print('Expecting 125 - got ' + str(copy_of_increment_x()))

```

The following will print
Expecting 124 - got 124
Expecting 125 - got 124

Is there a way to reference globals?