Support for generating pep484 .pyi stubs

[Static typing in python](https://realpython.com/python-type-checking/) (aka PEP-484) is gaining a lot of traction, with linting tools like [mypy](https://mypy.readthedocs.io/en/latest/) reaching maturity, and support in IDEs -- like PyCharm, VSCode, and Sublime -- for using pyi stubs to drive code inspection and completion. Stubs are particularly impactful for c/c++ bindings which are opaque to static inspectors.

pyi stub generation is already supported by other python binding generators like pybind11, shiboken, and sip.

Has this feature been discussed at all in boost-dev circles?

The issue names no repository files, tests, or entry points. Start by reviewing the linked PEP-484, mypy, and binding-generator references, then inspect the Boost.Python codebase to locate binding-generation entry points. Before implementation, clarify the supported stub scope and define tests or examples that establish when generation is complete.

Scritto dal modello di indicizzazione a partire dal testo della issue.