Improve typing and MIME hook API for inspector (#14342) Fixes https://github.com/ipython/ipython/issues/14339 ### Additions Adds `InfoDict` type to improve the typing of `info()` result. Adds missing `"subclasses"` to `info_fields` list (these were added to the field list in https://github.com/ipython/ipython/pull/11486 but we forgot to update `info_fields` variable at the time) - the newly added `InfoDict` type will ensure that this won't happen again. Adds `InspectorHookData` dataclass which is passed to the MIME hooks which now should expect a single argument. Having a single dataclass argument enables us to deprecate individual fields, or add new fields without breaking the existing hooks. The old hooks will still work (if any are out there since this mechanism got just added in the previous point version). ### Deletions A comment over `info_fields` gets deleted: - Contrarily to the comment (which is getting deleted in this PR), `info_fields` were not defining the order of display since at least 2015 (https://github.com/ipython/ipython/pull/7903 - I did not feel the need to go further in the history to find when exactly it happened). - Also contrarily to this comment, current Jupyter messaging spec does not define the contents of `info_fields` (I guess this was lost during IPython/Jupyter split), but the newly added `InfoDict` at least properly annotates their type (if you know where I can find the old IPython messaging spec with the descriptions I can add these as doc comments). Unused `cast_unicode` import gets deleted. If someone imported it from here... well they really should not have. ### Deprecations - mime hooks taking two arguments (`obj, info`)