[WIP] Add support for "requires" key in Python install config and allow extra keys beginning with "x-"

[NiklasRosenstein]

• Adds support for a "requires" install step in the Python install config. This allows you to specify requirements directly in the readthedocs config.
• Allow extra keys in the v2 configuration file if the key name starts with x-.

My motivation for this change is to reduce the number of files needed if

a) the requirements are needed only for readthedocs, and thus it makes sense to put them in the configuration directly, and
b) add the ability to enrich the readthedocs config with non-standard data for custom workflows

[stsewd]

Thanks for the PR. We are discussing this feature at #5572. I think we want to accept a list of packages, that way we nor the user need to worry about separating/escaping each argument.

python:
  install:
      - packages:
            - mkdocs>=1.1
            - sphinx

Multiple packages keys can be present of course. I have notices this in the issue for further dicsussion there, feel free to comment there with any other suggestion 👍

Also, you may want to add the x-option in another PR or open an issue for further discussion of that feature.

[NiklasRosenstein]

Hey @stsewd ! Thanks for your comments. I've submitted two separate PRs per your suggestion: #7288 #7289