Add edges() override in MultiGraph to support the "keys" argument

[iconchris]

Small fix to correct the typing for the edges() method of the MultiGraph class, which, adds a keys argument in order to differentiate multiple edges between the same pair of vertices.

Without this fix, code like:

for _sourceId, targetId, key, data in graph.edges(sourceId, data=True, keys=True):
  pass

...produces a mypy error like the following even though the code executes correctly:

graph.py:218: error: No overload variant of "__call__" of "OutEdgeView" matches argument types "str", "bool", "bool"  [call-overload]
graph.py:218: note: Possible overload variants:
graph.py:218: note:     def __call__(self, nbunch: None = ..., data: Literal[False] = ..., *, default: object = ...) -> OutEdgeView[Any]
graph.py:218: note:     def __call__(self, nbunch: Any | Iterable[Any], data: Literal[False] = ..., *, default: None = ...) -> OutEdgeDataView[Any, tuple[Any, Any]]
graph.py:218: note:     def __call__(self, nbunch: Any | Iterable[Any] | None, data: Literal[True], *, default: None = ...) -> OutEdgeDataView[Any, tuple[Any, Any, dict[str, Any]]]
graph.py:218: note:     def __call__(self, nbunch: Any | Iterable[Any] | None = ..., *, data: Literal[True], default: None = ...) -> OutEdgeDataView[Any, tuple[Any, Any, dict[str, Any]]]
graph.py:218: note:     def [_U] __call__(self, nbunch: Any | Iterable[Any] | None, data: str, *, default: _U | None = ...) -> OutEdgeDataView[Any, tuple[Any, Any, _U]]
graph.py:218: note:     def [_U] __call__(self, nbunch: Any | Iterable[Any] | None = ..., *, data: str, default: _U | None = ...) -> OutEdgeDataView[Any, tuple[Any, Any, _U]]

Thank you!

[github-actions]

According to mypy_primer, this change has no effect on the checked open source code. 🤖🎉

[srittau]

I have no experience with networkx, but the change looks sensible. Thanks!