Provide PEP 561 marker file

[dimbleby]

So that users of yangson can benefit from the typing.

[dimbleby]

Although the typing information is perhaps not currently accurate enough for this to be a good idea:

$ mypy -p yangson --ignore-missing-imports
...
Found 490 errors in 15 files (checked 18 source files)

Probably need to get this clean before exporting will be useful.

Is this something you're interested in? If so, I (or others) might be willing to chip away at it, slowly. But if you're not bothered about the type annotations, let's just close this down!