Typing for scrapy/utils

[wRAR]

Another batch of improved typing, mostly covering scrapy.utils.* (not everything there was processed yet but I think it's already quite large).

I have one question for @elacuesta: #3869 added scrapy.utils.datatypes.LocalWeakReferencedCache which returns None if "key is not weak-referenceable", and because of how scrapy.utils.misc.is_generator_with_return_value() uses it, that function can return None in addition to an expected bool. It even uses a pattern of putting a value into a dict and getting it back. So I wonder if None there has the same value as False (as the result of is_generator_with_return_value() is only checked with a simple if) and if it's fine to cast it to bool like I do in this PR.

[codecov]

Codecov Report

Merging #5925 (2f256d6) into master (85f5122) will increase coverage by 0.29%.
The diff coverage is 94.52%.

❗ Current head 2f256d6 differs from pull request most recent head 712ee98. Consider uploading reports for the commit 712ee98 to get more accurate results

@@            Coverage Diff             @@
##           master    #5925      +/-   ##
==========================================
+ Coverage   88.89%   89.19%   +0.29%     
==========================================
  Files         162      162              
  Lines       11254    11287      +33     
  Branches     1827     1833       +6     
==========================================
+ Hits        10004    10067      +63     
+ Misses        965      928      -37     
- Partials      285      292       +7     

Impacted Files	Coverage Δ
scrapy/utils/engine.py	77.27% <71.42%> (-6.94%)	⬇️
scrapy/utils/request.py	96.09% <77.77%> (-1.53%)	⬇️
scrapy/utils/display.py	93.10% <85.71%> (+0.24%)	⬆️
scrapy/utils/url.py	96.36% <86.66%> (-3.64%)	⬇️
scrapy/utils/decorators.py	76.66% <88.88%> (+1.66%)	⬆️
scrapy/utils/deprecate.py	93.05% <89.47%> (-2.40%)	⬇️
scrapy/utils/python.py	88.75% <95.83%> (ø)
scrapy/crawler.py	87.37% <100.00%> (+0.06%)	⬆️
scrapy/resolver.py	92.30% <100.00%> (+0.09%)	⬆️
scrapy/spiderloader.py	100.00% <100.00%> (ø)
... and 10 more

... and 2 files with indirect coverage changes

[wRAR]

collections.OrderedDict and weakref.WeakKeyDictionary are not generic in 3.7 and 3.8, the former can be replaced with typing.OrderedDict while the latter I think doesn't have that option, so I think I'll just remove specialization from it.