Previously wasn't defined in a way that is compatible with the protocol
`mypy` doesn't check this as nothing *was* annotated

- Uses standardised import checking/skipping
- Imports `InteractiveShell` from the module
- Replaces global scope with fixtures

Will look at fixing `mixins` after merging #3675

- `isinstance` check is unreachable and invalidates the `@overload` for `spec_to_mimebundle`
- Added note for the cause on remaining ignore

…ents

Some common themes:
- Testing with *known* invalid arguments
- Using `PluginRegistry.get()` without checking for `None`
- `SchemaBase.(to|from)_dict` ) allows more than just `dict`

Also adds a note for `alt.jupyter`, no plans to fix that