docs: 20.2 testing and migration guide

[brainwane]

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[brainwane]

notes from a conversation with Pradyun just now:

"Dependency resolution for" - remove. should be Installing From

Setups with 100+ packages

Using virtualenvs -- remove.

Output from "pip compile" when fed into pip like https://github.com/pypa/warehouse/blob/master/requirements/main.txt
hash-checking, pinned dependencies,

complex requirements file .... req files containing hashes

non-Latin localized filesystems, such as Japanese, Chinese, and Korean, and right-to-left such as Hebrew, Urdu, and Arabic

Linux within Windows

Continuous integration setups

add survey

[brainwane]

Nearly there - just need to add link(s) to the survey(s) and where to report bugs, and add links to the discussions about things being removed (e.g., support for constraints files).

I'll prefer a squash merge for this one since I have so many WIP commits.

[brainwane]

OK, I've now added a CHANGELOG/NEWS entry - 3-4 sentences: summarizing change, pointing to testing/migration guide, mentioning the rollout timeline. I believe this is ready to merge.

@pradyunsg should I care about the linter failing in CI? It passes locally.

[brainwane]

Oh goodness, how embarrassing for me, since you did not squash when you merged this, all my "WIP" commits are individually in the commit log. :(

[pradyunsg]

Oh no. I'm so sorry.