Add support for upcoming Scrapy 2.14

[wRAR]

Based on a local tox -e provider.

Breakages:

• downloader._get_slot() called in scrapy_zyte_api._middlewares._BaseMiddleware.slot_request() and in some tests no longer has a spider argument (we can reintroduce it if we really want)
• create_instance() called in tests (we already have a build_from_crawler() implementation for Scrapy versions that don't have it in scrapy_zyte_api.utils, we should just switch to it)
• HTTP10DownloadHandler used in the addon tests (I filed this earlier as #245, we can define our own dummy handler for this test I think)

Deprecations (some of them are from older versions):

• required spider argument of middlewares and pipelines and passing it to base class methods
• start_requests() in test spiders
• ExecutionEngine.close_spider() called in several places
• ExecutionEngine.download() in _SessionManager and ZyteApiProvider

[wRAR]

Analysis updated for current master with more deprecation removals