This directory stores symlinks to standard library modules and packages that are fully type-annotated and ready to be used in type checking of the rest of the stdlib or Tools/ and so on.
Due to most of the standard library being untyped, we prefer not to
point mypy directly at Lib/ for type checking. Additionally, mypy
as a tool does not support shadowing typing-related standard libraries
like types, typing, and collections.abc.
So instead, we set mypy_path to include this directory,
which only links modules and packages we know are safe to be
type-checked themselves and used as dependencies.
See Lib/_pyrepl/mypy.ini for an example.