gh-36736: avoid importing `DiGraph` from `sage.graphs.graph`
We remove the top level import of `DiGraph` from
`src/sage/graphs/graph.py` to prevent using `from sage.graphs.graph
import DiGraph`.
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URL: https://github.com/sagemath/sage/pull/36736
Reported by: David Coudert
Reviewer(s): Matthias Köppe
