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.

No files or tests are named. Start by tracing how the Python SDK is generated from dropbox-api-spec with stone and how releases are packaged; then determine where the generated mypy stubs should be published. Done means users can obtain stubs through the proposed PyPI package or typeshed and keep them updated with SDK releases.

Written by the indexing model from the issue text.