Remove reference of Pipenv + setup.cfg in README

[korikuzma]

On staging branch (9f64b22)

Note: there's also a Pipfile so you can skip the virtualenv steps if you'd rather use a Pipenv instance instead of virtualenv/venv. I have been sticking with the latter because Pipenv doesn't play well with entry points in development, but if you aren't editing them in setup.cfg, then the former should be fine.

We no longer have setup.cfg file and instead use pyproject.toml. It also looks like we removed Pipenv too, so no Pipfile.

[github-actions]

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