Improve the backwards compatibility for RetryMiddleware.EXCEPTIONS_TO_RETRY

[wRAR]

Fixes #6049 (with the code provided by the reporter)

[codecov]

Codecov Report

Merging #6050 (431fb01) into master (dba3767) will decrease coverage by 0.07%.
The diff coverage is 100.00%.

❗ Current head 431fb01 differs from pull request most recent head 800c1f1. Consider uploading reports for the commit 800c1f1 to get more accurate results

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6050      +/-   ##
==========================================
- Coverage   88.95%   88.88%   -0.07%     
==========================================
  Files         163      163              
  Lines       11567    11571       +4     
  Branches     1877     1876       -1     
==========================================
- Hits        10289    10285       -4     
- Misses        969      975       +6     
- Partials      309      311       +2     

Files Changed	Coverage Δ
scrapy/downloadermiddlewares/retry.py	100.00% <100.00%> (ø)

... and 1 file with indirect coverage changes