make debugger class configurable

[smacke]

In this PR, add fields compiler_class and debugger_class defaulting to the normal types. Kernel subclasses can override these to provide additional customizations.

[jasongrout]

Awesome, thanks! In full disclosure, we'd like this at Databricks (where Stephen and I both work) because we customize the debugger class.

[krassowski]

It seems fine, but not 100% sure as I get an error when trying to install kernel: #1309

[jasongrout]

It seems fine, but not 100% sure as I get an error when trying to install kernel: #1309

Following up here for completeness, it seems like David fixes this error in #1310

[Carreau]

Can this get rebased to trigger CI (I try to do it, but there is no allow push from maintainers).

[smacke]

Hey @Carreau just rebased + pushed

[Carreau]

There is one minor lint to fix (unused mypy ignore), and I restarted the test on 3.9.

I would actually be in favor of dropping 3.9 support (per https://scientific-python.org/specs/spec-0000/), and FYI IPython CLI already dropped 3.10.

[smacke]

@Carreau did you want me to fix those lint failures? They don't seem related with this change but happy to do it anyway

[Carreau]

No, it was just a "the failures are know and fixed on master". I haven't been involved in ipykernel in a while and was reluctant to merge, but I think that's ok.