All ``Arist`` now carry wai-aria data

It is now possible to attach `wai-aria

<https://developer.mozilla.org/en-US/docs/Web/Accessibility/ARIA/Roles>`__ role

information to any `~matplotlib.artist.Aritist`. These roles are industry the

industry standard for providing accessibility mark up on the web. This

information can be used by downstream applications for providing accessible

descriptions of visualizations. Best practices in the space are still

developing, but by providing a mechanism to store and access this information

we will enable this development.

There are three methods provided:

- `~matplotlib.artist.Aritist.set_aria` which will completely replace any existing roles.

- `~matplotlib.artist.Aritist.update_aria` which will update the current roles in-place.

- `~matplotlib.artist.Aritist.get_aria` which will return a copy of the current roles.

We currently do no validation on either the keys or the values.

Matplotlib will use the `'aria-label'` role when saving svg output if it is

provided.