gh-89610: Add .pyi as a recognised extension for IDLE on macOS (GH-95393)

This allows opening stub files by double clicking on them in the Finder.

Co-authored-by: Terry Jan Reedy <tjreedy@udel.edu>
(cherry picked from commit 06fc2491)

Co-authored-by: Erlend Egeberg Aasland <erlend.aasland@protonmail.com>