Added py.typed marker file, minor type change

This is my first contribution to this repo, even if it is a small one. I tried to read up on the contribution guidelines but I apologize in advance if I missed anything.

What does the merge request do?

Inkex already has plenty of great typing information, but without the py.typed file mypy won't use any of it when it's imported by a third-. Adding this file lets third-party extension developers (such as Inkstitch, where I'm hoping to better use it) benefit from all that type-checking information.

It also includes a minor type change to make sure that the selection attribute is exposed on SvgInputMixin.svg.

Summary for release notes

This isn't really a change that impacts "interested users" and not developers. How about: "3rd-party extension authors can now use inkex's type information."

Checklist

• Add unit tests (if applicable) (They aren't, I don't think.)
• Changes to inkex/ are well documented
• Clean merge request history