Python module issues

[matveitolochko]

Submodules are currently broken.
See:

• ModuleNotFoundError: No module named 'supermodule.submodule'; 'supermodule' is not a package PyO3/pyo3#759
• Can't pickle <class 'builtins.RustClass'>: attribute lookup RustClass on builtins failed PyO3/pyo3#1517 (comment)
• https://gist.github.com/ethanhs/fd4123487974c91c7e5960acc9aa2a77

[matveitolochko]

Fixed in bbc66fe

Explanation:

1. We create the submodule via PyModule::new
2. Call that the function that represents the submodule (providing py, so this module)
3. Then we add the submodule to the parent module. (m.add_submodule(module_name))
4. Finally, run a python script to add the submodule to sys.modules.

I created this macro to do so easier:

macro_rules! pymod {
    ($module_name: ident -> $submodule_func: path, $module_path: literal, $py:ident, $m: ident) => {
        let $module_name = PyModule::new($py, ::core::stringify!($module_name))?;
        $submodule_func($py, $module_name)?;
        $m.add_submodule($module_name)?;
        pyo3::py_run!(
            $py,
            $module_name,
            ::core::concat!(
                "import sys; sys.modules['",
                $module_path,
                "'] = ",
                ::core::stringify!($module_name),
            )
        );
    };
}

[matveitolochko]

Also added these same docs in 1774431