[stubgenc] fixes and typos (#9877)
* [stubdoc] fix typo
* [stubgenc] Add Callable to _DEFAULT_TYPING_IMPORTS
* [stubgenc] Fix parsing signatures with empty argument list
* [stubgenc] Replace NoneType with None
* [stubgenc] Skip pybind11-specific internal attributes
* [stubgenc] Test stub generation on pybind11-mypy-demo project
