Can we export exceptions?

[kngwyu]

In this PR I added a new macro named export_exceptions, which inserts

#[pymethods]
fn _pyo3_exceptions(_py: pyo3::Python, m: &pyo3::types::PyModule) -> pyo3::PyResult<()> {
    use pyo3::type_object::PyTypeObject;
    m.add("PyBorrowError", pyo3::pycell::PyBorrowError::type_object())?;
    m.add(
        "PyBorrowMutError",
        pyo3::pycell::PyBorrowMutError::type_object(),
    )?;
    Ok(())
}

to exension, so that our custom exceptions can be imported to Python code.

But somehow except PyBorrowError fails oddly like

____________________________________________________________ test_catch_borrrow ____________________________________________________________

    def test_catch_borrrow():
        m = Mutable()
        try:
            m.invalid_borrow(m)
            assert False
        except PyBorrowError:
            pass
        except Exception as e:
>           raise e

tests/test_pycell_exceptions.py:13: 
_ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ _ 

    def test_catch_borrrow():
        m = Mutable()
        try:
>           m.invalid_borrow(m)
E           _pyo3_exceptions.PyBorrowError

and also isintance(e, PyBorrowError) returns False.

[kngwyu]

This test succeeds.

[kngwyu]

These tests fail.

[birkenfeld]

Maybe it's easier to define the module in Python (some global code running once when PyO3 initializes), and use the exception class from there like other Python exceptions?

[kngwyu]

Closed since it's outdated now, though exceptions should eventually get able to be exported