Breaking change in `TypeVarType` causes an error during typecheck

[fjarri]

Mypy 1.4 includes PR python/mypy#14872 that added a positional deault parameter to the mypy.types.TypeVarType constructor. plugin.py::takes_callable_and_args_callback() invokes it without that argument, causing mypy to crash.

This seems to be fixed by passing AnyType(TypeOfAny.from_omitted_generics) as an additional argument, but this will make trio-typing incompatible with mypy<1.4. Thoughts on how to proceed?