[red-knot] Type narrowing for assertions (take 2) (#17345)
## Summary
Fixes #17147.
This was landed in #17149 and then reverted in #17335 because it caused
cycle panics in checking pybind11. #17456 fixed the cause of that panic.
## Test Plan
Add new narrow/assert.md test file
Co-authored-by: Matthew Mckee <matthewmckee04@yahoo.co.uk>
