Fix -o - broken in 2.6

[Laerte]

Closes: #5444

This happen because argparse module treats the - as prefix chars: https://github.com/python/cpython/blob/main/Lib/argparse.py#L2205:L2207

Since we cannot change the internal implementation of _parse_optional i created a class that inherits from ArgumentParser class and override the method with our use case calling the default _parse_optional.

[codecov]

Codecov Report

Merging #5445 (7659c7d) into 2.6 (23537a0) will increase coverage by 0.00%.
The diff coverage is 100.00%.

❗ Current head 7659c7d differs from pull request most recent head e17d831. Consider uploading reports for the commit e17d831 to get more accurate results

@@           Coverage Diff           @@
##              2.6    #5445   +/-   ##
=======================================
  Coverage   88.77%   88.78%           
=======================================
  Files         163      163           
  Lines       10666    10671    +5     
  Branches     1818     1820    +2     
=======================================
+ Hits         9469     9474    +5     
  Misses        922      922           
  Partials      275      275           

Impacted Files	Coverage Δ
scrapy/cmdline.py	67.96% <100.00%> (+1.30%)	⬆️

[Gallaecio]

@Laerte Could you please rebuild your branch on top of 2.6? This is a change we would like to release in 2.6.2.

There is probably a better way, but this should do the trick:

git remote add upstream https://github.com/scrapy/scrapy.git  # unless you have already setup this remote before
git fetch upstream
git checkout upstream/2.6 -B master  # Warning: this destroys your branch contents locally; mind that if you add further commits
git cherry-pick 7d20ce65479cdbe39fc17b7cb2dc99415c670dae  # Cherry-picks your commit here; if you had added more commits before doing this, you should note their hashes and do the same with them
git push -u origin master -f  # Pushes your new master branch, which is upstream/2.6 + your commit, to your master branch, dropping the 2 upstream master commits that are not in upstream 2.6

[Laerte]

@Gallaecio Done! Tks. 🥇

[Gallaecio]

Thanks!