gh-3243: support pickle & deepcopy

[helpmefindaname]

Closes gh-3243

A suggestion how we could support pickle and deepcopy without having to create multiple spans etc.

The idea is to use __getitem__ to create spans and relations by indexing: sentence[0:5] to create a span, sentence[span_a, span_b] to create a relation.

Downside would be, that those who still instantiate Span or Relation manually, would receive unexpected results.

@dobbersc @alanakbik what do you think?

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