CSV item exporter does not flush on close

Description

The csv item exporter does not flush on close.

Steps to Reproduce

1. Create pipeline with an CSV item exporter
2. only export few items (one or two)
3. Let spider close/finish

Expected behavior: All exported items are in the CSV Files

Actual behavior: Files with few items are empty (not even a header)

Reproduces how often: 100%

Versions

Scrapy : 2.3.0 lxml : 4.5.2.0 libxml2 : 2.9.10 cssselect : 1.1.0 parsel : 1.6.0 w3lib : 1.22.0 Twisted : 20.3.0 Python : 3.8.2 (default, Jul 16 2020, 14:00:26) - [GCC 9.3.0] pyOpenSSL : 19.1.0 (OpenSSL 1.1.1f 31 Mar 2020) cryptography : 2.8 Platform : Linux-5.4.0-7642-generic-x86_64-with-glibc2.29

Example pipline.py

from itemadapter import ItemAdapter
from scrapy.exporters import CsvItemExporter

class CricketcrawlerPipeline:
    def open_spider(self, spider):
        self.name_to_exporter = {}

    def close_spider(self, spider):
        for exporter in self.name_to_exporter.values():
            exporter.finish_exporting()

    def process_item(self, item, spider):
        exporter = self._exporter_for_item(item)
        exporter.export_item(item)
        return item

    def _exporter_for_item(self, item):
        name = item['name']
        if name not in self.name_to_exporter:
            f = open(f'{item.folder}/{name}.csv', 'wb')
            exporter = CsvItemExporter(f)
            exporter.start_exporting()
            self.name_to_exporter[name] = exporter
        return self.name_to_exporter[name]

[wRAR]

Another consideration, the file-like object that is passed to the exporter doesn’t necessarily support the flush method.

[scientes]

Yes i’ve used the docs as basis. Maybe we should Update the documentation to close the files.