Incidentally, I should have mentioned in my original announcement:
pip 20.3 turned the new resolver on by default for Python 3 users. When users use pip 20.3 in a Python 2 environment, the old dependency resolver is still the default. (Further discussion: https://github.com/pypa/pip/issues/9194 .)
pip 21.0 in January will remove Python 2 support: https://pip.pypa.io/en/latest/development/release-process/#python-2-support .