Zuban Update

[davidhalter]

• Result changes for recent releases
• Wrap one assert_type(..., Never) in a if bool(), because some type checkers might mark the rest of the module as unreachable (commit 28a7aed)
• Adds a way to only run the tests for a specific type checker e.g. python main.py --only-run mypy (commit 2dc2643)

I felt like the last two changes made sense anyway and shouldn't be controversial, so I added them.