gh-36277: `sage.graphs`: some care with return ... else statements in `graph.py`, `digraph.py` and `bipartite_graph.py`
We change this kind of statements in `graph.py`, `digraph.py` and
`bipartite_graph.py`:
```py
if ....:
return ...
else:
return ...
```
to
```py
if ....:
return ...
return ...
```
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### :hourglass: Dependencies
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URL: https://github.com/sagemath/sage/pull/36277
Reported by: David Coudert
Reviewer(s): David Coudert, Matthias Köppe
