Publish PEP 561 type information

[sirosen]

I currently have an application using marshmallow-oneofschema and linting under mypy. As I tune up mypy strictness, I noticed errors about "subclassing Any".

marshmallow-oneofschema does not publish annotations right now. As a result, OneOfSchema isn't known to type-checkers (at least mypy) as a subclass of Schema. It also means that the OneOfSchema attributes are unrecognized.

I don't think much needs annotation. The primary change needed is the inclusion of py.typed, as in other marshmallow-code projects. I may open a few PRs when I get time to add more annotations.