Fix OntologyGraph type annotations - breaking change

[ielis]

Correct the type annotations on OntologyGraph.get_children, OntologyGraph.get_parents, OntologyGraph.get_ancestors, OntologyGraph.get_descendants.

Previously, we promised iterables while the methods in fact provide iterators. This may have led to a bug if the iterator was used, e.g. within a nested loop:

hpo: MinimalOntology = ...

parents = hpo.graph.get_parents('HP:0001250')
for _ in range(5):
  for parent in parents:
    print(parent)

The loop above would, in fact, print only during the first iteration of the outer loop, and the iterator would be "consumed" for the remaining 4 iterations.