ruff

18eaa659 - [ty] Introduce a representation for the top/bottom materialization of an invariant generic (#20076)

[ty] Introduce a representation for the top/bottom materialization of an invariant generic (#20076) Part of #994. This adds a new field to the Specialization struct to record when we're dealing with the top or bottom materialization of an invariant generic. It also implements subtyping and assignability for these objects. Next planned steps after this is done are to implement other operations on top/bottom materializations; probably attribute access is an important one. --------- Co-authored-by: Carl Meyer <carl@astral.sh>