Limit tests to Twisted < 21

[Gallaecio]

@kmike @wRAR @elacuesta I figured out this package upgrade was the issue so far.

According to the Twisted 21.2.0 release notes:

trial now allows the @unittest.skipIf decorator to specify that an entire test class should be skipped. (#9829)

This confuses me greatly, as what is failing now are tests that were already using this feature that is supposed to be new in Twisted 21.2.0.

I’ll continue looking into it later.

Fixes #5020

[codecov]

Codecov Report

Merging #5022 (4fe26ae) into master (f95ebd8) will decrease coverage by 0.16%.
The diff coverage is n/a.

@@            Coverage Diff             @@
##           master    #5022      +/-   ##
==========================================
- Coverage   88.01%   87.84%   -0.17%     
==========================================
  Files         158      158              
  Lines        9726     9726              
  Branches     1433     1433              
==========================================
- Hits         8560     8544      -16     
- Misses        911      928      +17     
+ Partials      255      254       -1     

Impacted Files	Coverage Δ
scrapy/robotstxt.py	75.30% <0.00%> (-22.23%)	⬇️
scrapy/core/downloader/__init__.py	92.48% <0.00%> (+1.50%)	⬆️

[wRAR]

Have you seen #5020 ?

[Gallaecio]

🤦 I should never create PRs or tickets before checking all the existing ones.

[Gallaecio]

Maybe we should limit the Twisted version for now, to not block pull requests, and figure out what the root issue is later.

[wRAR]

Sure