DelayExstensionVertex typing weirdness

[Christian-B]

after #1342

class DelayExtensionVertex
@Property
def _delay_splitter(self) -> SplitterDelayVertexSlice:
if TYPE_CHECKING:
return cast(SplitterDelayVertexSlice, self._splitter)
else:
return self._splitter

Issue is that to import a SplitterDelayVertexSlice causes circular imports
the cast call actually does nothing but does please the typer

if TYPE_CHECKING is only true when tryping

keep this hack or find a better solution