`importlib.metadata.PackageMetadata` now has a `get` method

[duckinator]

Previously, #7767 was closed because at the time the get method of PackageMetadata was considered an implementation detail, and @AlexWaygood said to open a new issue if that situation changes.

python/importlib_metadata#444 made it part of the API.

[Akuli]

If I understand importlib_metadata's README correctly, this will be included for the first time in Python 3.13.

[srittau]

I'd be ok with fudging our rules a bit here, if in practice the get method was always present. But I have only superficial experience with PackageMetadata.

[Akuli]

I see another reason why we should add .get() on all Python versions. Let's say you have an implementation of PackageMetadata that doesn't have .get(). You probably want to get type checker errors for it as soon as possible, because it will no longer be a valid implementation of PackageMetadata in Python 3.13.