[fix master] fix AbstractSet-related mypy failure
Fixes the error seen in this build:
Which originated from this diff: https://dagster.phacility.com/D7499
I'm not sure how the error snuck through, because the last build on that diff succeeded. Maybe a race condition?
Test Plan: bk
Reviewers: alangenfeld, max
Reviewed By: alangenfeld, max
Differential Revision: https://dagster.phacility.com/D7549