Engine: deprecations and type hints

[elacuesta]

As discussed earlier @wRAR

Type hints adapted from #3559

Deprecated methods:

• ExecutionEngine.open_spiders: I didn't check the history, but I suspect this probably comes from the time when multiple spiders could run in a single crawl. That hasn't been the case for a long time, so it's not necessary to have a plurally-named property with only one element.
• ExecutionEngine.has_capacity: same as the above, the engine only handles one spider slot, which can be checked directly. Not called from outside the engine.
• ExecutionEngine.schedule: Not called from outside the engine, deprecated as per Requests scheduled when idle never go through the spider middlewares #542 (comment)

Also, the spider argument is deprecated for the spider_is_idle, crawl and download methods, the spider instance saved in the ExecutionEngine.spider after calling open_spider is used instead. I would also like to deprecate passing a spider argument to close_spider, but that would be more complicated because of the order of the positional arguments, let's leave it for a future PR.

[codecov]

Codecov Report

Merging #5090 (f6a95b0) into master (5b78a64) will decrease coverage by 0.06%.
The diff coverage is 81.59%.

❗ Current head f6a95b0 differs from pull request most recent head ab29cbc. Consider uploading reports for the commit ab29cbc to get more accurate results

@@            Coverage Diff             @@
##           master    #5090      +/-   ##
==========================================
- Coverage   88.25%   88.19%   -0.07%     
==========================================
  Files         162      162              
  Lines       10374    10392      +18     
  Branches     1506     1510       +4     
==========================================
+ Hits         9156     9165       +9     
- Misses        948      952       +4     
- Partials      270      275       +5     

Impacted Files	Coverage Δ
scrapy/extensions/memusage.py	50.00% <0.00%> (+1.08%)	⬆️
scrapy/utils/engine.py	52.63% <ø> (ø)
scrapy/core/engine.py	84.12% <82.16%> (-3.38%)	⬇️
scrapy/core/scraper.py	87.86% <100.00%> (ø)
scrapy/downloadermiddlewares/robotstxt.py	100.00% <100.00%> (ø)
scrapy/pipelines/media.py	97.16% <100.00%> (ø)
scrapy/shell.py	67.96% <100.00%> (ø)