Support installing a Poetry dependency group

Poetry 1.2 will be adding added support for dependency groups. It would be ergonomic if you could install a dependency group (as a whole) into a nox session.

• https://python-poetry.org/docs/managing-dependencies/#dependency-groups

Consider a Poetry group:

[tool.poetry.group.lint.dependencies]
flake8 = "*"
flake8-bugbear = "*"
flake8-broken-line = "*"
flake8-comprehensions = "*"
pep8-names = "*"

The cooresponding nox session could be simplified to:

def test(session: nox_poetry.Session):
    session.poetry.install_group("lint")
    ...

This avoids the DRY problem of listing all of the lint dependencies twice, once in a Poetry group and again in a nox session.

[johnthagen]

poetry export supports --only

poetry export --help

Description:
  Exports the lock file to alternative formats.

Usage:
  export [options]

...

--only=ONLY         The only dependency groups to include. (multiple values allowed)

So I think this would be a relatively easy way to get a requirements.txt file out for a specific group.

[thedrow]

@edgarrmondragon Go ahead 😃