Provide machine-readable optional dependencies?

Congratulations on 7.5!

Downstream on conda-forge, noted that markdown is needed for the test suite to run. Exciting!

However, it would be lovely to have a centralized place for this kind of information, and future optional dependencies changes. This would certainly make it easier for downstreams such as conda-forge, but could also provide a actionable user messages

Design ideas:

One approach would be to leverage python's optional dependencies, which could be captured in various schemes with the robotframework[extra1,extra2] syntax.

This might look like:

setup(
    # ...
    extras_require={
      "gfm": [
          "pygments",
          "robotframework[markdown]", 
      ],
      "markdown": [
         "markdown",
      ],
      "utest": [
         "robotframework[markdown]",
         # ....
      ]
      #... and pyte, xml, yaml, and...
    },
)

pretty verbose, but, selfishly: i like seeing diffs of sorted lists )

Or, with fancy new pyproject.toml, which can separate out unexported names in dependency-groups:

[project.optional-dependencies]
gfm = [
  "robotframework[markdown]", 
  "pygments"
]
markdown = [
  "markdown",
]
# ...

[dependency-groups]
utest = [
  "robotframework[markdown]",
  # ... actual unique test deps
]

this would also work if moving all of the setup.py content over to pyproject.toml, with a tiny setup.py of __import__('setuptools').setup() which wouldn't require any docs changes... though maybe some automation

I'd be happy to work up a strawman PR for either approach, if desired.