mypy

a7d4c67d - Document PEP 585, 563, 604 and more (#9763)

Document PEP 585, 563, 604 and more (#9763) Fixes #8629, fixes #8523. This creates a new page to document issues arising from discrepancies between the runtime and annotations. I felt this was better, rather than force-fitting things into existing pages and "common issues", for instance, it prevents us from having to explain PEP 563 in several different places. I do still list the runtime errors you'd get in the "common issues" page to preserve SEO :-) "String literal types", "Class name forward references", and "Import cycles" are basically the same as where they were copied over from. This also factors out the documentation of PEP 604 that I promised when merging that PR (it seemed pretty verbose, particularly for the "kinds of types" page). It's also a good place to document PEP 613, when we get around to supporting that. Resolves #9856. Co-authored-by: hauntsaninja <>