[PyCDE] Enable client code to type check properly

As of today, it is impossible to use the pycde API in a way which pleases python type checkers (e.g. mypy). Make this possible. Will require API changes. Not sure that this is even possible given that we parameterize classes/functions so much.

Different from #3581 in that it doesn't require generating pybind11 stubs. That was only for internal development.