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

Merging #6274 (84056f4) into master (861646f) will decrease coverage by 0.03%.
The diff coverage is 37.50%.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6274      +/-   ##
==========================================
- Coverage   88.82%   88.79%   -0.03%     
==========================================
  Files         161      161              
  Lines       11863    11867       +4     
  Branches     1918     1919       +1     
==========================================
  Hits        10537    10537              
- Misses        979      982       +3     
- Partials      347      348       +1     

Files	Coverage Δ
scrapy/commands/bench.py	100.00% <100.00%> (ø)
scrapy/spiders/__init__.py	88.23% <28.57%> (-5.52%)	⬇️

[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.