Fix typing issues for typed Scrapy.

[wRAR]

Remaining issues are related to scrapy/scrapy#5989 and to typed Spider.parse().

[codecov]

Codecov Report

Merging #166 (dede201) into master (aba2b74) will increase coverage by 0.01%.
The diff coverage is 100.00%.

❗ Current head dede201 differs from pull request most recent head 5e6238d. Consider uploading reports for the commit 5e6238d to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #166      +/-   ##
==========================================
+ Coverage   85.85%   85.87%   +0.01%     
==========================================
  Files          14       14              
  Lines         813      814       +1     
==========================================
+ Hits          698      699       +1     
  Misses        115      115              

Files	Coverage Δ
scrapy_poet/downloadermiddlewares.py	100.00% <100.00%> (ø)
scrapy_poet/injection.py	98.54% <100.00%> (+0.01%)	⬆️
scrapy_poet/page_input_providers.py	100.00% <100.00%> (ø)
scrapy_poet/spidermiddlewares.py	100.00% <100.00%> (ø)
scrapy_poet/utils/__init__.py	97.82% <100.00%> (ø)
scrapy_poet/utils/testing.py	77.16% <ø> (-0.70%)	⬇️