Provide mypy stubs for the Python SDK

**Why is this feature valuable to you? Does it solve a problem you're having?**
Type checking of Python code can be incredibly helpful to write readable and maintainable code.

**Describe the solution you'd like**
Currently, these stubs can be mostly automatically generated from https://github.com/dropbox/dropbox-api-spec using stone. However, it would be convenient if you could make those stubs available as a separate package on ~PyPI~ typeshed which gets updated with new releases of the SDK.