Failure with serializing/deserializing using Python `pickle`

[aalok-sathe]

I think something may be broken when serializing (pickle-dumping) and loading back conllu-parsed sentences.

File "python3.9/site-packages/conllu/models.py", line 103, in extend
    self.metadata.update(iterable.metadata)
AttributeError: 'TokenList' object has no attribute 'metadata'