sagemathgh-38189: Improve DiGraph neighbor iterators
    
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Currently, in the Digraph class, the two methods `neighbor_out_iterator`
and `neighbor_in_iterator` are just wrappers around low-level methods
`_backend.iterator_out_nbrs` and `_backend.iterator_in_nbrs`. To build
the iterator, the low-level iterator is first converted into a set and
then converted back into an iterator using `iter()`. It does not seem
efficient.

This PR proposes to use `yield from` instead (see the very short commit
9a89251)

As this modification changes the order of the output, the output of 4
tests need to be reorder (see commit 305b933). One can check that the
lists contains the same elements, but not in the same order.




### 📝 Checklist

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- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
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<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#38189
Reported by: cyrilbouvier
Reviewer(s): David Coudert, grhkm21

sagemathgh-38189: Improve DiGraph neighbor iterators
    
<!-- ^ Please provide a concise and informative title. -->
<!-- ^ Don't put issue numbers in the title, do this in the PR
description below. -->
<!-- ^ For example, instead of "Fixes sagemath#12345" use "Introduce new method
to calculate 1 + 2". -->
<!-- v Describe your changes below in detail. -->
<!-- v Why is this change required? What problem does it solve? -->
<!-- v If this PR resolves an open issue, please link to it here. For
example, "Fixes sagemath#12345". -->

Currently, in the Digraph class, the two methods `neighbor_out_iterator`
and `neighbor_in_iterator` are just wrappers around low-level methods
`_backend.iterator_out_nbrs` and `_backend.iterator_in_nbrs`. To build
the iterator, the low-level iterator is first converted into a set and
then converted back into an iterator using `iter()`. It does not seem
efficient.

This PR proposes to use `yield from` instead (see the very short commit
9a89251)

As this modification changes the order of the output, the output of 4
tests need to be reorder (see commit 305b933). One can check that the
lists contains the same elements, but not in the same order.




### 📝 Checklist

<!-- Put an `x` in all the boxes that apply. -->

- [x] The title is concise and informative.
- [x] The description explains in detail what this PR is about.
- [ ] I have linked a relevant issue or discussion.
- [ ] I have created tests covering the changes.
- [ ] I have updated the documentation and checked the documentation
preview.

### ⌛ Dependencies

<!-- List all open PRs that this PR logically depends on. For example,
-->
<!-- - sagemath#12345: short description why this is a dependency -->
<!-- - sagemath#34567: ... -->
    
URL: sagemath#38189
Reported by: cyrilbouvier
Reviewer(s): David Coudert, grhkm21