Solve constraints from typevar upper bounds

[hauntsaninja]

Fixes #13220

[hauntsaninja]

Well, mypy hated that.

I tried the following without success:

• scoping it to just when upper_bound is builtins.object
• moving the addition of the constraint to unify_generic_callable (which is where we need it)
• then scoping that one just to when upper_bound is builtins.object
• then removing the part in the solver where if there's a top type and no bottom type it picks the top type (as opposed to say, UninhabitedType)