Make sure `monkeypatch.undo()` is always called

[webknjaz]

While reading the docs, I noticed that in one place (in the fixture), monkeypatch.undo() is not wrapped with try/finally, but the CM method in the class, it is. It seems this could lead to weird behaviors so I figured I'd offer this patch seeing that it should improve things. Not sure if a change note is needed, feel free to request it :)

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• [ ] Include new tests or update existing tests when applicable.
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[The-Compiler]

The teardown part of fixtures is always called, even if there was an exception in the test - so I'm afraid I don't really see how this changes anything over what the current behavior already is.

[The-Compiler]

Taking the freedom to close this, as I believe the current code already works correctly, and I don't see any benefit in changing it.