chore: update get maintainers to use new JSON API field

[art1f1c3R]

Summary

The updated PyPI JSON API now includes a field "ownership" that containers the maintainers of the package in the JSON result. This PR updates the PyPI Registry code to use this instead of parsing the package HTML page.

Description of changes

The "ownership" field containers the organization associated with a package, and then each user and role. For example, black@26.1.0 has this:

"ownership": {
        "organization": null,
        "roles": [
            {
                "role": "Owner",
                "user": "JelleZijlstra"
            },
            {
                "role": "Owner",
                "user": "ambv"
            },
            {
                "role": "Owner",
                "user": "cooperlees"
            },
            {
                "role": "Owner",
                "user": "willingc"
            },
            {
                "role": "Owner",
                "user": "zsolzsol"
            }
        ]
    }

It is much neater and reliable to use this as opposed to parsing the HTML page of a PyPI package. This PR updates the PyPIPackageJsonAsset to have the get_maintainers_of_package function as part of it instead of the PyPIRegistry object, since it is part of the JSON. It then updates all references and uses accordingly.

Checklist

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• I have tested my changes and verified they work as expected.