Fix import_task_before_choice event for singleton imports

[marksdestiny]

Description

This PR fixes a bug which caused the import_task_before_choice event not to be sent for singleton imports.

I suppose this was just an oversight, as this behavior is not mentioned in the documentation.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[github-actions]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.