Flickr-photos-api

* The `url` of a `CommonsInstitution` is now optional rather than required.

* Change the `UserInfo` type so it can be subclassed again (this broke in v2.10.0).
This doesn't change the output/behaviour of the library.

* Internal refactoring for Flickr Foundation purposes; no public-facing changes.

* Add a `location: str | None` to the `UserInfo` type.

When you look up a user with `get_user(user_id)` and that user has a public location, it will be returned in the response.

If you look up a user who doesn't have a location, or their location is private, this value will be `None`.

* Add a `pro_account_expires: datetime.datetime` to the `UserInfo` type.

When you look up a user with `get_user(user_id)` and that user has Flickr Pro, you'll be able to see when their Flickr Pro subscription expires.

If you look up a user who doesn't have Flickr Pro, you won't get the `pro_account_expires` field.

* Remove the `get_buddy_icon_url(user_id)` method.

Instead, call `get_user(user_id)` and read the `buddy_icon_url` parameter from the response.

If you already have a result from `get_user(user_id)`, use that to save yourself an API call.