Allow latest versions of conda/mamba/ in .readthedocs.yml

[tdegeus]

What's the problem this feature will solve?

Often one does not care on which version the documentation is build. It would be good if one could accept a default latest version.

Describe the solution you'd like

version: 2

sphinx:
  configuration: docs/conf.py

build:
  os: ubuntu-latest
  tools:
    python: mambaforge-latest

conda:
  environment: environment.yaml

Alternative solutions

An alternative would be to have a bot open PRs on the source's repository if a version changes. Currently a lot of people have to do a lot of manual work for something that they likely don't care about

Additional context

[stsewd]

Hi, for latest on build.os, see #8861.

For mamba, since we support python: 3, I think we can support an alias for the other tools.

[ericholscher]

We've just shipped a version of this, so if you'd like to test it out, it should now be live. The version that latest will point to is the most recent in the docs: https://docs.readthedocs.io/en/stable/config-file/v2.html#build-tools-python