Updating the message when the user hits a conflict w/ installed packages

Environment

• pip version: master today.
• Python version: all
• OS: all

Description

We’ve decided to update the error message presented to a user, when the final result set of packages will contain dependency conflicts.

Context: #8513, #7744 and a recent team meeting regarding the new resolver rollout.

Expected behavior

The behavior has to be changed in two ways, each related to a different part of the resolver rollout plan:

• The legacy resolver would present a message nudging users to realize that the upcoming new resolver might behave differently (see #8513)
• The new resolver would present a message nudging users “hey, do you want the resolver to always be strict!? tell us!” (see https://github.com/pypa/pip/issues/7744#issuecomment-651411121)

How to Reproduce / Output

To get the current message, run:

$ pip install six==1.10.0
$ pip install cherrypy==11.0.0
Collecting cherrypy==11.0.0
  Downloading CherryPy-11.0.0-py2.py3-none-any.whl (435 kB)
     |████████████████████████████████| 435 kB 5.3 MB/s 
Requirement already satisfied: six in /Users/pradyunsg/.virtualenvs/tmp-7af976d02abc202/lib/python3.8/site-packages (from cherrypy==11.0.0) (1.10.0)
Collecting portend>=2.1.1
  Downloading portend-2.6-py2.py3-none-any.whl (5.1 kB)
Collecting cheroot>=5.2.0
  Downloading cheroot-8.3.0-py2.py3-none-any.whl (86 kB)
     |████████████████████████████████| 86 kB 5.9 MB/s 
Collecting tempora>=1.8
  Downloading tempora-3.0.0-py2.py3-none-any.whl (14 kB)
Collecting more-itertools>=2.6
  Using cached more_itertools-8.4.0-py3-none-any.whl (43 kB)
Collecting jaraco.functools
  Downloading jaraco.functools-3.0.1-py3-none-any.whl (6.7 kB)
Collecting pytz
  Using cached pytz-2020.1-py2.py3-none-any.whl (510 kB)
ERROR: cheroot 8.3.0 has requirement six>=1.11.0, but you'll have six 1.10.0 which is incompatible.
Installing collected packages: more-itertools, jaraco.functools, pytz, tempora, portend, cheroot, cherrypy
Successfully installed cheroot-8.3.0 cherrypy-11.0.0 jaraco.functools-3.0.1 more-itertools-8.4.0 portend-2.6 pytz-2020.1 tempora-3.0.0

Note the line starting with “ERROR:”.

[brainwane]

it would be useful to be able to tell users when the new resolver will become default

I think we should say “October 2020”.

[pradyunsg]

/cc @ei8fdb @nlhkabu for their inputs on the messages to present to the users in both these cases.