Docs: add instructions on config to default to new resolver

Some people would like to set an environment variable manually on the command line, or something in pypi.rc or pip.conf, so they can tell pip to default to the new resolver #6536 #988 and test it more easily, without having to manually include --use-feature=2020-resolver each time. Example via Twitter:

It would be handy to be able to enable the new deps resolver via an env var. This would allow testing it in conjunction with tools such as poetry and pipenv.

@Julian in IRC logs suggested editing pip.conf and daturkel says this worked:

[install] use-feature = 2020-resolver

We should decide whether we want to support this, and then if we do, we should add a line about it to the how-to-test docs.

[akaihola]

Note that if you modify configuration using

pip config set global.use-feature 2020-resolver

it isn’t possible to disable the resolver simply using

pip config unset global.use-feature

Instead, you’ll need

pip config unset --user global.use-feature

I see this behavior on Fedora 30, Python 3.7 and pip 20.2.

[McSinyx]

I can’t reproduce this on my Debian machine with CPython 3.8 and pip 20.2. pip config unset global.use-feature does unset the feature, but I notice that it unsets the entire option and AFAIK there’s yet to be a way to deselect just one feature. I’m wondering if we want to support that, and if so I’ll file a separate ticket tracking it.

@akaihola, I wonder what pip config debug says on your system.