Python requests post JSON raw data in SharePoint

I want to update some property in a SharePoint ListItem.

I'm using this code:

body = '{"__metadata": {"type": "SP.Data.Documentos_x0020_compartidosItem"}, "Tarea": "TP 15"}'

header = {'Authorization': 'Bearer ' + bearer + '', 'Accept': 'application/json;odata=verbose', 'X-HTTP-Method':'MERGE','content-type': 'application/json', 'IF-MATCH'
: '*'}

and I make this call:

requests.post("https://[sharepoint URL]/_api/web/lists/GetByTitle('Documentos')/items(232)",json=body,headers=header).json()

but I get always the same answer:

{'error': {'code': '-1, '
                   'Microsoft.SharePoint.Client.InvalidClientQueryException',
           'message': {'lang': 'es-ES',
                       'value': "Se encontró un nodo 'PrimitiveValue' "
                                'inesperado al leer del lector JSON. Se '
                                "esperaba un nodo 'StartObject'."}}}

Value translation: "An unexpected 'PrimitiveValue' node was found when reading from the JSON reader. It was expected a 'Startobject' node"

Could anyone give me any clue about this issue?

Previously I built similar request on Postman and it worked succesffully:

Thanks and kind regards