Improve typing for Spider.parse().

[wRAR]

Based on https://stackoverflow.com/questions/77160087/correct-typing-of-a-method-without-a-fixed-signature

Should remove most of the mypy errors for parse() in user spiders (if they use older mypy Spider.parse() will appear untyped so no override error either). Doesn't help with scrapy-poet's callbacks that narrow down Response to DummyResponse: scrapinghub/scrapy-poet#179

This should probably have typing tests, I'll do that later.

[codecov]

Codecov Report

Attention: Patch coverage is 37.50000% with 5 lines in your changes are missing coverage. Please review.

Project coverage is 88.79%. Comparing base (563ecbe) to head (84056f4).
Report is 16 commits behind head on master.

❗ Current head 84056f4 differs from pull request most recent head 050aef3. Consider uploading reports for the commit 050aef3 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6274      +/-   ##
==========================================
+ Coverage   84.88%   88.79%   +3.90%     
==========================================
  Files         161      161              
  Lines       11927    11867      -60     
  Branches     1810     1919     +109     
==========================================
+ Hits        10124    10537     +413     
+ Misses       1523      982     -541     
- Partials      280      348      +68     

Files	Coverage Δ
scrapy/commands/bench.py	100.00% <100.00%> (+4.44%)	⬆️
scrapy/spiders/__init__.py	88.23% <28.57%> (-11.77%)	⬇️

... and 95 files with indirect coverage changes

[wRAR]

This could be useful in other places as it's a general type for a spider callback. I didn't think about that further though.

[wRAR]

Actually we should use it for Request.callback

[wRAR]

Looks like we can't, probably because of wrong covariance when assigning a specific callback to a request, I need to investigate this separately.