resolver ran for more than an hour on Klein

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Klein has ben intermittently and unreliably producing EXTREMELY long tox environment installs.

Klein version c5b0b2b05d2616ae98a3e00451460b461628db9e

Twisted trunk version 7cf6c8bc320ac5fd96b4784f6feb932ea819856d

I ran tox -vvvv -r -e coverage-py39-twtrunk and watched it spin for about an hour. All other environments (besides the PyPy ones which are broken for other reasons eventually pass, usually after about 6-7 minutes (which is still 10x the amount of time it actually takes to run the tests).

I don't have dep trees or anything here because attempting to dig out exactly which pip invocation is the problem is tough; I need to use something like https://github.com/pglass/tox-pip-version to inject the pip version and I don't have the time to fiddle with this that much that right now, so I wanted to get this filed first.

[pradyunsg]

This seems to be a duplicate of #9187.

You can also use --use-deprecated=legacy-resolver (or the env-var equivalents) for now, until we figure out a good way to deal with the issue of behaviour with extremely tricky dep trees with conflicts.