augmented assignment statements #5314 update

[alexpdev]

Changed the 3 instances mentioned in Issue #5314

Fixes #5314, closes #5321.

[alexpdev]

Since the changes in this PR didn't alter any functionality I didn't add any tests.

I know a pull request has already been submitted on this issue, but after reviewing the diff for that PR I decided to start over.

Let me know if I got anything wrong.

[codecov]

Codecov Report

Merging #5322 (0287e18) into master (28eba61) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5322   +/-   ##
=======================================
  Coverage   88.59%   88.59%           
=======================================
  Files         163      163           
  Lines       10616    10616           
  Branches     1558     1558           
=======================================
  Hits         9405     9405           
  Misses        936      936           
  Partials      275      275           

Impacted Files	Coverage Δ
scrapy/core/downloader/handlers/http11.py	94.11% <100.00%> (ø)
scrapy/core/http2/stream.py	91.37% <100.00%> (ø)

[elfring]

Thanks for your small source code improvement.