Reuse `mypy_primer` to check issue reproductions?

[A5rocks]

As the title says, mypy_primer could totally detect diffs in mypy output for repros for issues on mypy -- this could help find issues that a PR closes. However, this would require having an up-to-date listing of them, which is/will probably be a challenge.

Perhaps as an MVP, just get every code block from every open issue tagged "bug" and run mypy over them...?