ImportError: cannot import name 'soft_unicode' from 'markupsafe'

[Gallaecio]

Looking at the CI jobs of #4267, pallets/markupsafe#282 is hitting us in some scenarios.

[wRAR]

Right, I've restarted tests for master some time ago to check that and forgot to check the results.

https://github.com/scrapy/scrapy/actions/runs/1855091848
https://github.com/scrapy/scrapy/actions/runs/1855091860

[wRAR]

So I guess this is because old mitmproxy, which we use for older Python versions and in other cases, requires old Flask which is affected.

[wRAR]

I think it should be enough to pin markupsafe in the same cases that pin mitmproxy (also in conda).