Create a hook that implements PEP 843

[aroberge]

PEP: https://peps.python.org/pep-0843/

Discussion: https://discuss.python.org/t/pep-843-export-statement-for-dry-re-exports/108687

Also add, as an option (or different example, soft_export_keyword?)

export name = ...
export def function ...
export class Name ...