Add EntitySchema data type

[eloiferrer]

Hi @LeMyst,

I am seeing a new error when using basic functions such as item.get(). This is ultimately triggered by the new EntitySchema data type.

The error can be triggered by:

wbi.item.get('Q5')

and it happens for any entity that uses property P12861. This is the error:

  File "/usr/local/lib/python3.11/site-packages/wikibaseintegrator/entities/item.py", line 127, in get
    return ItemEntity(api=self.api).from_json(json_data=json_data['entities'][entity_id])
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/wikibaseintegrator/entities/item.py", line 143, in from_json
    super().from_json(json_data=json_data)
  File "/usr/local/lib/python3.11/site-packages/wikibaseintegrator/entities/baseentity.py", line 161, in from_json
    self.claims = Claims().from_json(json_data['claims'])
                  ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/wikibaseintegrator/models/claims.py", line 128, in from_json
    data_type = [x for x in BaseDataType.subclasses if x.DTYPE == claim['mainsnak']['datatype']][0]
                ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^
IndexError: list index out of range

This PR adds this new data type so that these kind of errors are avoided. Maybe you prefer another approach if you don't plan to support this new data type. In any case, it would be helpful to fix the issue somehow. Thanks!

[LeMyst]

Hello @eloiferrer
Thanks for your PR, I'm gonna review this and probably add more things to it this afternoon.

[LeMyst]

Are my changes good for you @eloiferrer ?

[eloiferrer]

Yes, it looks great!