fix node import themes.py

[Tara-Lakshmipathy]

Current implementation throws NameError: name 'Node' is not defined Node class is imported only when the code is being type-checked, but not when it's being executed which causes the error.

[liamhuber]

lgtm, but you can also get it with from __future__ import annotations at the top of the file. Although the future import is supposedly not necessary from 3.11 onward, in this case when combined with TYPE_CHECKING I find I do still need it -- not sure about 3.12.

Since Node is not actually used outside the type hint, the annotations import is my preferred solution because (a) it makes it marginally clearer how the import is used in this file to keep it wrapped in TYPE_CHECKING (subjective!), and (b) it can lead to speedups (objective). In this case, pyiron_workflow.node is getting imported by the other imports anyhow, so the speedup is non-existent and thus your solution here is objectively fine.