Allow 'callback' key in keyword arguments for request callbacks

[elacuesta]

Fixes #5237

Use Deferred.addCallbacks instead of Deferred.addCallback. Modified the test case to work with this patch but fail with the current code in master.

[elacuesta]

The failed pylint check is because of the f-strings, being worked on at #5246.

[codecov]

Codecov Report

Merging #5251 (9d3e7fb) into master (7d557f8) will not change coverage.
The diff coverage is 100.00%.

❗ Current head 9d3e7fb differs from pull request most recent head 67cf329. Consider uploading reports for the commit 67cf329 to get more accurate results

@@           Coverage Diff           @@
##           master    #5251   +/-   ##
=======================================
  Coverage   88.51%   88.51%           
=======================================
  Files         163      163           
  Lines       10592    10592           
  Branches     1524     1524           
=======================================
  Hits         9375     9375           
  Misses        943      943           
  Partials      274      274           

Impacted Files	Coverage Δ
scrapy/core/scraper.py	87.86% <100.00%> (ø)