fix(testing): pytest.approx correctly take account Mapping keys order to compare them

[cmnemoi]

Hello,

This PR fixes an issue with pytest.approx where the error message incorrectly reported all elements as mismatched when comparing mappings with different key orders, even when only some values differed.

The original code paired values by position rather than by key. This caused incorrect mismatch reporting when dictionary keys were in different orders.

This closes #12444.

Checklist

• I have included new tests or updated existing ones where applicable.
• I have created a new changelog file in the changelog folder, named <ISSUE NUMBER>.<TYPE>.rst.
• I have added myself to AUTHORS in alphabetical order.
• I have added text like “closes #XYZW” to the PR description.
• I have allowed maintainers to push and squash when merging my commits.
• I have included documentation when adding new features. (Not applicable here – this is a bug fix.)

[cmnemoi]

Hello @bluetech, can I have a review here ? It should be fast.

[cmnemoi]

Hello @nicoddemus, thanks for your review for #13818

Here is a similar contribution to take a look to when you have the time

[nicoddemus]

Let's access the key directly, because the previous checks already ensure both dicts have the same set of keys.

Suggested change

	for approx_key, approx_value in approx_side_as_map.items():
	other_value = other_side.get(approx_key, None)
	for approx_key, approx_value in approx_side_as_map.items():
	other_value = other_side[approx_key]

[nicoddemus]

Given the output message is central to the issue, please rewrite this test using pytester and verifying the output itself.

Also, let's use a few more keys in both dictionaries to cover more ground. I suggest:

expected = {"a": 1, "b": 2, "c": 3, "d": 4}
actual = {"d": 4, "c": 5, "a": 8}