Actually flip the switch on the new resolver

[pradyunsg]

We're rolling out the new resolver as part of pip 20.3, and we need to flip the flags so that the new resolver is used by default in pip, with the old resolver being available behind --use-deprecated-feature=legacy-resolver as decided in #8371.

Roughly, the things that need to be done for this are:

• Update the conditionals in the codebase to switch the default behavior.
• Update the tests, to correctly handle the new resolver vs old resolver (run with new resolver by default).
• Update the documentation, to denote that the new resolver is the default.

[sbidoul]

@pradyunsg @brainwane @pfmoore @uranusjr following #8664 (comment) and the next in the same thread, I'd suggest considering to never make the new resolver the default for python 2.

Especially since 20.3 is the last release to support python 2 it may be better to avoid creating unnecessary troubles for a version that we'll stop supporting soon.

[uranusjr]

I am +0 on the proposal. It has always seemed awkward to me that the default resolver switch happens only one major version before Python 2 support is dropped entirely.

[pradyunsg]

I think that'll work well, especially since we're dropping support in the next release (which, given our approach to branching+releases) means we can start dropping Python 2 support in master immediately after 20.3 is released.

[brainwane]

@sbidoul I think you have enough consensus here to go ahead -- could you start a PR to make that change?

[sbidoul]

@brainwane I'd love to but I'm afraid I don't have the bandwidth to work on that in the short term.

[pradyunsg]

With #9044 and #9019, this is now done! ^>^