List of fields incorrectly accessed for dataclass items

[tadejsv]

Description

If I make a dataclass item and want to export to csv, I get this error:

...
  File "/home/tadej/miniconda3/envs/main/lib/python3.7/site-packages/scrapy/exporters.py", line 251, in _write_headers_and_set_fields_to_export
    self.fields_to_export = list(item.fields.keys())
AttributeError: 'CompanyItem' object has no attribute 'fields'

The problem stems from here

https://github.com/scrapy/scrapy/blob/master/scrapy/exporters.py#L243-L253

There should be an additional if case checking if the item is of type dataclass, and then accessing the fields differently, perhaps as

[field.name for field in fields(item)]

[elacuesta]

Indeed, seems like we missed that one in #3881. Thanks for the report.