include py.typed file to make typing annotations available for library users

[karlicoss]

Hi! Mistletoe has type hints, but sadly they aren't available when you're using it as a library unless you include py.typed file (for example see https://blog.whtsky.me/tech/2021/dont-forget-py.typed-for-your-typed-python-package )

I checked the change locally: before the change running mypy on my project would result in this

src/promnesia/sources/markdown.py:7:1:7:1: error: Skipping analyzing "mistletoe": module is installed, but missing library stubs or py.typed marker  [import-untyped]
    import mistletoe
    ^
src/promnesia/sources/markdown.py:8:1:8:1: error: Skipping analyzing "mistletoe.span_token": module is installed, but missing library stubs or py.typed marker  [import-untyped]
    from mistletoe.span_token import AutoLink, Link
    ^

After the change that works! And now it's catching possible typos etc when using mistletoe

[coveralls]

coverage: 94.207%. remained the same
when pulling 99c2ef4 on karlicoss:pytyped
into 14b1876 on miyuchina:master.

[pbodnar]

@karlicoss, thank you! 👍 Testing took me a while because I hit some related issues, but this seems to work just fine in the end. :)