Add exceptions to modules

After creating a new exception using:

create_exception!(my_module, MyException, ExceptionClass);

It is natural that I would want to make it available as an import within my python module:

from my_module import MyException

However, in order to do this, I had to do some deep Kung Foo action:

#[pymodule]
fn my_module(py: Python, m: PyModule) {
    m.add("MyException", py.get_type::<MyException>())?;
}

It would be nice to have an easier API for this:

#[pymodule]
fn my_module(_py: Python, m: PyModule) {
    m.add_exception::<MyException>()?;
}

Start by reading the create_exception! macro and the #[pymodule] API shown in the issue to understand how exception types are currently exposed. Implement an add_exception API that supports the demonstrated import, then verify that MyException can be imported from my_module and that existing exception creation remains functional.