Fix spans weak ref in doc copy

[svlandeg]

Description

Doc.copy() would corrupt the doc reference stored in doc.spans, because the copied spans would keep pointing to the old doc, which would get garbage-collected at some point, eventually leading to dead references.

Proposed solution: allow Spangroups.copy() to optionally define a new Doc object, so that Doc.copy() sets the correct new Doc.

Types of change

bug fix

Checklist

• I have submitted the spaCy Contributor Agreement.
• I ran the tests, and all new and existing tests passed.
• My changes don't require a change to the documentation, or if they do, I've added all required information.

[svlandeg]

I guess an alternative solution would be to have a doc.spans setter which makes sure that you can't set an incompatible SpanGroups object on a given Doc - because I can imagine this bug popping up in other places as well.

[honnibal]

Ah good catch, thanks.