Include a py.typed marker to benefit from type annotations

[bluenote10]

First of all, thanks a lot for writing and maintaining auraloss -- it is great to have a common package for these functions!

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auraloss seems to contain reasonable type annotations, at least for a certain subset of functions.

Unfortunately, the library doesn't ship with a py.typed marker (c.f. PEP 561, which means that type checkers and IDEs don't make use of the type annotations. It would be nice to add a py.typed to really benefit from the type annotations, especially since using type checking is becoming more and more a standard.

If that would be fine I can open a PR to add it, and perhaps also integrate a type check into the CI, and extend the existing type annotations a little bit. What do you think?