test_integration_downloader_aware_priority_queue raises exception

[Lukas0907]

Description

The test tests/test_scheduler.py::TestIntegrationWithDownloaderAwareInMemory::test_integration_downloader_aware_priority_queue raises the following exception:

ERROR    scrapy.core.engine:engine.py:308 Stats close failure
Traceback (most recent call last):
  File "/home/lukas/Projects/3rd/scrapy/.tox/py/lib/python3.8/site-packages/twisted/internet/defer.py", line 654, in _runCallbacks
    current.result = callback(current.result, *args, **kw)
  File "/home/lukas/Projects/3rd/scrapy/scrapy/core/engine.py", line 328, in <lambda>
    dfd.addBoth(lambda _: self.crawler.stats.close_spider(spider, reason=reason))
  File "/home/lukas/Projects/3rd/scrapy/scrapy/statscollectors.py", line 48, in close_spider
    self._persist_stats(self._stats, spider)
  File "/home/lukas/Projects/3rd/scrapy/scrapy/statscollectors.py", line 61, in _persist_stats
    self.spider_stats[spider.name] = stats
TypeError: unhashable type: 'list'

The exception is only logged by scrapy.core.engine and hence swallowed by py.test. I discovered it accidentally when using the log_cli = true pytest setting.

Steps to Reproduce

1. Set log_cli = true in pytest.ini
2. Run tox -e py -- tests/test_scheduler.py -k test_integration_downloader_aware_priority_queue

Versions

Scrapy 2.1.0 / master

Additional context

The issue should be simple to fix, I will prepare a PR for it.