Remove legacy resolver

[ssbarnea]

No description provided.

[atugushev]

I propose the following plan:

1. switch the default value of --resolver to backtracking in 7.0
2. remove completely legacy resolver code in 8.0 as soon as pip removes its legacy resolver

[ssbarnea]

I propose a fast-tracked approach, remove it in 7 but release 7 as pre-release for some time, so early adopters can make use of it. Anyway, that does not mean I will fight your proposal.

I would also vote for removing support for py37 in v7, even if is not EOL. To give you an idea, I also removed support for py38 in several projects and libraries. If anyone is still stuck with older versions, there is nothing preventing them from using an older version of pip-tools anyway, and we would at least be able to move faster and refactor our code to make use of improvements in python and other deps.